Astronotes-Jan-Feb-2009

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Jan-Feb, 2009 Astronotes

Vol. 48, No. 1 ISSN 0048-8682 Jan-Feb 2009
Astronotes-Feb2009-Cover.jpg
December 20th Sun in Ottawa,
by Richard Taylor


Ottawa Centre Meeting Report - December 5, 2008

Recorder's Notes by Estelle Rother

With Chris Teron at the controls, president Chuck O'Dale began the evening by chairing the annual business meeting. The minutes from the 2007 business meeting were approved. This was followed by several reports. In the president's report, Chuck noted the highlights of year. The Ottawa Centre is the second largest with 404 members. Monthly meeting averaged an attendance of 150. Chuck then thanked the members of the executive and the appointed positions for their contributions to the Centre. Since the 1970's there have been many discoveries: gamma ray bursts, more than 150 extra solar planets have been found, Mars and Mercury have been mapped, rovers explored Mars, hydrocarbon lakes have been detected on Titan, and ice volcanoes were seen on Enceladus. Many new words have been added to the English language including url,gps, and html. And we have a new meaning for Windows. What will we see thirty years from now, in 2038? Chuck concluded his report with 'Thank you. I've had a blast'.

Hans Brouver presented the treasurer’s report. Mr. Potter has completed the audit for 2007 and he will prepare the 2008 audit. He recommended the Centre keep a record of its assets. His recommendation for SMARTscope is use it or lose it. There was a small loss for the year. The Centre is considering ways to cut costs so there will be no member surcharge. There is a reserve in GICs that can be used for SMARTscope and the IYA. The financial statement was approved. Hans commented that in the outlook for this year, things are in good shape.

The Ted Bean Telescope Library receives rental income of $10 per month for scope rental. Some older equipment was sold and this income was used to purchase a UHC nebula filter. The library needs some high magnification eyepieces.

The Paul Comision Observer of the Year was awarded to Gary Boyle. Gary has been astrocasting and sharing his observations with everyone. The Astronotes Article of the Year was awarded to Mike Earl for his July/August article "Hickson 50 and the Question of Magnitude'. A Service Award was presented to Debra Ceravolo in October for her work on the GA and the IYA.

The councillor positions were all acclaimed. Paul Harrison is our new president. And the business meeting was adjourned. Brian McCullough then chaired the rest of the meeting.

Tim Cole presented Ottawa Skies for December. On December 11, the moon would pass through the stars of the Pleiades. On December 15, the moon would pass through M44, the Beehive cluster. And near sunset on the last day of the year, there would be a Kodak moment with Mercury near Jupiter and Venus would be close to the moon.

When Gordon Webster began observing about a year ago, Brian suggested he keep pa log and make sketches of what he saw. He presented Maintaining an Astronomical Observing Log. Even though he is an artist, he initially hesitated to do sketches. But now he feels he has wasted an observing session if he has not made at least one sketch. His first problem was what to use as a logbook. Using a computer (his laptop is old enough to vote) did not allow for sketching and Gordon understands paper and pencil. His first logbook was a spiral bound project sketchbook with blank space at the top for sketches and a lined lower half for notes. An Internet search yielded several types of log pages but these asked for more technical information than Gordon was prepared to deal with and the sketching area was either too small or too large. So he created his own looses leaf pages. This allowed him to add pages for different observing sessions as well as drawings he had done from his sketches. He also has a section for observing challenges, space for detailed charts, and observing target lists. Circles for drawing areas are traced using the right sized glass, bowl or plastic tub. High tech people can try tracing a CD or DVD. Gordon's observing target list allows him to include a sketch of what he is looking for as well as the object, type, how to find it and remarks. Experience has showed that preplanned observing sessions are more rewarding. Instead of taking the whole log book, Gordon uses a folding clipboard with a variety of log pages. He also uses a small clip light with a lens he has painted red. He reminded us we are doing these sketches for ourselves. We do not have to show them to anyone. All we are really doing is putting dots inside a circle and we can all draw dots. Start with double stars and open clusters and then move on to really faint fuzzies. Begin with the brightest stars to define the field. For nebulosity, use the side of a pencil lead and smudge it with your finger. Think of the eyepiece as a clock and note positions relative to the hours. When drawing faint objects you are looking carefully for details to sketch. After a few attempts, you will feel confident to sketch more difficult objects.

Glen Ledrew talked about Improving My Home Made Binoculars. He originally used 35 mm objectives from another binocular because they gave a sharp image. The eyepieces were taken from another binocular, the Bushnell extra wide. Glen likes their 85-degree apparent field of view. He later replaced the objectives with the 50 mm objectives from the same Bushnell binoculars. These did not give as sharp an image as the original 35 mm objectives. Glen's day job is figuring telescope mirrors so this summer he decided to change the objectives' surface shape. The original 50 mm objectives showed spherical aberration and an extra defect. When Glen was finished polishing the front surfaces, the resolution had improved 2 1/2 times. Multiple stars are now distinct and star clusters have pin point stars instead of appearing as little fuzzies.

After a short break and the distribution of door prizes, the meeting continued with an enjoyable presentation of 3D images from Mars and lunar images including the areas of the Apollo15 and Apollo 17 landing sites.

Simon Hanmer has given many talks about the geology of the rocky planets and the moons of the outer planets. These talks always used two-dimensional Power Point slides. For Brian's last night as program chair, Simon wanted to do something different. So he presented a 3D animated Tour of the Inner Solar System as if we were really there, flying through space and visiting the planets and their moons. The Software Bisque program does 3D animation of already rendered 3D objects in their right place and time. The program can be run manually or as a stand-alone animation. The idea of the program is that you are in a spaceship, space is big and things happen slowly in space. Time can be speeded up. As you move around in space, there is no such thing as up. But earthlings like to see things in a certain orientation so Simon sometimes needed to correct the ship's attitude so things would appear as we expected to see them. Once Simon launched his presentation, he could not stop it. He provided a description of what we were seeing.

Members' Observations was the last topic of the meeting. Brian started with a Santa/Moon conjunction. Paul Comision showed images of NGC0136, NGC0628 and NGC0772. Mike Earl showed images of M44 (the Eagle Nebula) and M20 (the Trifid Nebula in Sagittarius). Nova Ophiuchi 1 was discovered on May 25. This slow nova was still detectable in December. A second nova decayed within 60 days. Mike showed light curves of both novas. He also showed comet Cardinal and the December 1 conjunction of the Moon, Venus and Jupiter. Bob Olson showed NGC253 and NGC288, both low in the sky. Attilla reported on the November deep sky challenge. He used a 6-inch refractor and a 24 mm eyepiece to see NGC253.

December Observing Challenges:

Deep Sky: Attilla's pick G1/Mayall II in the halo of M31 is 10 arc seconds across. Although it can be seen in a 10- inch scope using 200 power, a 15-inch is recommended. Fourteen additional clusters can be seen using a 16-inch scope.

Lunar: Brian's pick Observe the Northern Gateway, the strait allowing passage from Mare Serenitatis westward into the Mare Imbrium and Southern Line of the lunar tramway.

Brian concluded the meeting with a short retrospective of the past 23 meetings. He thanked Chris Teron and Tim Cole for their technical support. His final act as Centre chair was to turn over the keys of office (the box for door prize tickets). Thanks to Ann and Art Fraser for the after meeting refreshments.

Ottawa Centre Meeting Report – January 9, 2009

Recorder's Notes by Estelle Rother

New Centre meeting chair, Attilla Danko, opened the January meeting with web master Richard MacDonald at the controls. In the Ottawa Skies talk, Brian McCullough highlighted what we could see in the January sky. Rob Cardinal discovered Comet Cardinal in October 2008. It is now found in Ursa Minor and is moving fairly quickly. Now is also a good time to observe Saturn's moon Mimas. See page 336 of the Observer's Handbook for details. Venus, Mercury and Jupiter are the evening planets for the month. Comet Lulin (C/2007 N3) is visible low on the horizon in the southeast sky between 5:30 am and 6:45 am. And on January 10, the full moon would be at perigee and it is the largest full moon of 2009.

Meteor observing is Pierre Martin's passion. He also enjoys dark skies, attending star parties, photography and public outreach. In his talk, Observing Meteors and Other Madness, Pierre told us about his activities of the past few years. He owns a coffin. No, not what you think it is. This one, used for meteor observing, is made of sheets of plywood held together with door hinges that fold around his lawn chair. Aluminium reflective tarps over the top provide some protection from the cold. Sometimes, Pierre is lucky enough to observe an aurora and he showed several photographs. After one exceptional aurora, the sky became very dark and he took some images of the southern Milky Way. In 2001, the Leonid meteor shower was spectacular. Also in 2002, there were some intense meteor storms where the hourly rate reached 3000 for a short time. But the moon was bright and the meteors tended to be fainter than in 2001. Not all showers are as spectacular. Minor meteor showers are poorly understood and data from amateurs helps astronomers better understand their behaviour. plotting meteors on a star chart is useful to determine the structure of the radiant. Mosquitoes were a problem during the 2004 Bootes shower. Pierre's solution was a ThermaCELL, a mosquito repellent that actually works. The 2006 Persied shower counts were reduced to 20 per hour because of the full moon. But Pierre set up a camera to take automatic exposures while he enjoyed the show from his lawn chair. 2006 was also supposed to be a good year for the Geminids. There was only one problem: the weather map showed all clouds. So Pierre drove 1800 kilometres to Tennessee to see the shower, where he observed rates greater than 100 an hour. It was definitely worth the drive. He is also interested in comets since they are the origin of meteor showers. He photographed comet Holmes and comet McNaught. Pierre then talked about the Orionids. This is normally a moderate shower with a peak hourly rate of between 15 and 20. Recently, the Orionids have been very active with many fireballs. The 2007 shower was spectacular and the night after the predicted peak was even better. Pierre was also involved in public outreach for the February 2008 lunar eclipse. In the summer, he created a panorama of the Milky Way. On the same evening, he observed noctilucent clouds. Meteorite dust might be responsible for the formation of these upper atmosphere clouds. He was also fortunate to have a good view of the Perseids. And finally, for the long weekend in September, an impromptu star party took place at a site called Nirvana.

Ken Whitnall then presented Hydrogen Alpha Solar Observing. Four years ago, Ken bought his first solar filter and discovered that unlike many objects we observe at night, the sun is very dynamic. The photosphere is at the end of the conduction zone and is about 500 kilometers thick. In this area the temperature is only 5500 Kelvin. The next layer is the chromosphere. In this transition zone between the photosphere and the corona the temperature increases from 6000 Kelvin at the top of the photosphere to 50,000 Kelvin at the top of the chromosphere. The temperature rises to 1 million Kelvin in the corona. About 70% of the solar material is hydrogen. Oxygen, nitrogen and calcium are among the other elements found in the sun. In the chromosphere, nitrogen plasma is suspended above magnetic field lines. Excited electrons in the chromosphere give off red light. this red ring can be seen during a solar eclipse. Dark absorption lines in the solar spectrum are created when electrons in atoms absorb a photon. These lines allow scientists to analyze the chemical composition of the sun and to determine the temperature, density and wind speed.

Why study the sun in hydrogen alpha? A hydrogen atom has one electron orbiting its nucleus. When this electron is excited, it moves from a low energy orbit to a higher energy orbit. When an electron in the n3 level drops back to the n2 level, it emits a photon at 656 nanometers. This hydrogen alpha line is in the visible spectrum. Several features can be seen in hydrogen alpha light. Prominences are seen on the limb of the sun. Dark filaments are the same solar activity as prominences but are seen on the solar disk. There are quiet and active type prominences and filaments. These features occur where clouds of hydrogen plasma coalesce along magnetic field lines across the surface of the sun. Granulation is a mottling of the solar surface. Granules occur at the top of the convection zone and can be between 1000 and 2000 kilometers long. White clouds, known as plages, can be seen in areas of high magnetic activity and are also associated with sunspots. Spicules are spikes of superheated gas that are 500 kilometers in diameter and are found 5000 kilometers above the solar surface. They stream at speeds of 22 kilometers/second and last 5 to 10 minutes. Spicules are formed by sound waves with the same 5-minute period.

Members' Observations was the next presentation. Roman Dzioba, an Ottawa Centre member, submitted images from Australia of the Southern Cross and the Large Magellanic Cloud. Paul Comision showed images of NGC0559, an open cluster in Cassiopia, NGC0891 in Andromeda and NGC7332 and NGC7339 in Pegasus. Peter Hayman showed the 'X'on the lunar terminator and a more complete lunar mosaic than a previous image he had shown. Bob Olson showed a moon mosaic; G1, a Mickey Mouse ears globular cluster 2.5 light years away; a mosaic of M42, the Orion nebula; and M61. Rolf Meier showed a 5-day-old moon. You can only see detail on the terminator so Rolf plans to create a lunar mosaic using images of the terminator. He also showed an image of Venus, Mercury and Jupiter taken on January 3.

Simon Hanmer presented Mike Worth's new Lunar Images. Mike is a Centre member who is now in Baja Mexico and is considered to be the best amateur lunar photographer. Simon first reminded us what we are looking at when we look at the moon. The white areas are anorthosite. These rocks have the same composition as the original coffee mate. The dark areas are basalt lava. This basalt is the same stuff as the ocean basins on Earth. Mike's images focus on the Imbrium impact basin. Impact basins formed early in the history of the moon. They are much larger than the impacts that formed the craters. The multi-ringed impact basins then filled with lava hundreds of millions of years later. In the images, you can see a time sequence from older to younger and see how the moon has evolved through time.

The first image showed the northern side of the Imbrium basin in the area of impact crater Plato. The Alpine Mountains are the flooded (by lava, not water) remnants of the outer ring of the multi-ringed impact basin. Ejecta was thrown out of the hole when the major impact occurred. The Alpine Valley on the northern ridge is a rift valley. Impact craters pockmark the Alpine Hills. The valley has a flat bottom with few craters. Complex craters tend to be larger than simple craters. They usually have a central peak and ridges and terraces around the outside. The complex crater Plato does not have a central peak. The slump features we see are landslides. The walls of the crater collapsed inwards into the hole. Plato has a flat floor with few craters. This tells us the floor is young and is filled with lava that came up through the floor. You can see a valley on the northeast edge of Plato. This collapsed lava tube is over 100 kilometers long and 1 kilometer across. The outer part of the tube froze but lava continued to flow through the middle of the tube. After the lava flowed out onto the mare floor, the tube was empty so there was nothing to hold it up and the tube collapsed. The important thing is the sequence. The anorthosite formed about 4.5 billion years ago. At about 3.8 billion years ago, major impacts formed the multi-ringed basins. Then more impacts occurred. Several hundred million years later, as the interior of the moon heated up by radioactive breakdown, basalt formed and came up through the weakest parts of the lunar crust. Wrinkle ridges formed in the basin lavas. Some think they are the edges of different lava flows. Others think the lava draped over faults. Then there are younger simple impact craters. The Alpine Valley has a straight side and a complicated side. There is a squiggly valley going down the middle. It is an erosional lava river that is hundreds of kilometers long. You can see most of this with a relatively small telescope.

The area of Plinius is at the boundary between Serenity and Tranquility. Scars are caused by impact ejecta from a crater somewhere else, which has travelled across the surface of the moon. The scarring is flooded by basalt. There are 2 different kinds of lavas. Long narrow rilles only occur in the older, dark lavas. In the younger light colored lavas, you have compressional wrinkle ridges. Older craters are partially flooded. There are both simple and complex craters.

Posedonius is on the eastern rim of Serenity. The crater is flooded with basalt. When the lavas came in to fill the impact basin, they either came up through the floor or they came in through the western edge of the crater. Posedonius is a large complex crater. Slumping along its edges are pre flooding. There are curved and straight fractures on the crater floor. As basalt came through the floor, it pushed the floor up, stretching it and forming ridges. One fracture starts out as a rift and turns into a ridge. The fracture has changed direction. So you get stretching or shortening depending on where you are.

Lacus Mortis is on the north side of Serenity. Many think it is a large, flooded crater. But there is a problem. The feature has a hexagonal shape. There are no fractures parallel to the walls. Geologically, it is difficult to make a pattern of hexagonal fractures. The complex crater Burg that has a central peak, slumps and landslides has impacted Lacus Mortis.

Crater Cauchy is on the east side of tranquility. Several flooded craters are seen a distance from the edge of the impact basin. This tells us that the lavas are thin there. First the impact basin formed. It then is peppered by further cratering and then the basin is flooded by lava. There are 2 linear features, one on either side of Cauchy. One is a simple rift with a kink. Simon would call it a segmented fault, which is 2 rifts, slightly offset from one another. But he does not know why they would join. The other feature goes from being a rift to being a ridge. There are several volcanic domes with summit craters. This volcanism is as old as the mare lavas, which filled the impact basins.

Simon concluded with the message that anyone can see the geology on the moon and see how the various areas of the moon actually formed through time.

Astromart is the most popular online place to buy and sell astronomical equipment. Richard Harding talked about the Rules of Acquisition as they apply to astro buying online. He told us that free advice is seldom cheap. Take the time to read the terms and conditions for Astromart.. You cannot list on Astromart if you also list the item on eBay. Astronomers treat their gear with care. You should know what things are worth. Do your homework and know the real Canadian and US price of the item you are interested in. And research past prices on Astromart. Try to buy Canadian, especially on high end items. The Canadian government does not tax items worth less than $150. Boxes, caps and papers are important on any item. Be friendly. And do not be afraid to offer less than the asking price.

Richard listed a few Do's:

  • Be patient
  • Be honest
  • Do not over represent your item
  • If it is too good to be true, it probably is not true
  • Avoid sellers with a bad rating He also listed some Do Not's:
  • Assume you are both on the same page
  • Ignore negative feedback
  • EVER accept Any shipping other than US postal service
  • Forget that this service is not a business
  • Fail to read the terms of use

Paul Harrrison, the new president of the Ottawa Centre, gave a short presentation. He moved to Ottawa 5 years ago and has been the vice president for the past 2 years. Council has 3 main roles: operations of the organization, public outreach and the organization. The Centre has 3 members on national council. Public outreach this year includes participation in the Interantional Year of Astronomy. The Centre also participates with the Museum of Science and Technology and holds star parties.. Paul then introduces members of council.

Mike Moghadam provided an update on Outreach Events for 2009. For the IYA Canada launch at the museum, the Centre had a telescope display, IYA literature and a light pollution displa. Mike was successful in his application for a Galileo lecture. Dr. Roberto Abraham, from the University of Toronto department of astronomy and astrophysics, will speak about Cosmic Dawn and Monster Telescopes. Mike is also planning other events including an IYA poster contest and an additional prominent speaker in April or May. Other events include 100 hours of astronomy: a 24 hour webcast from many observatories, the celebration of the 40th anniversary of the first manned lunar landing, live webcast of the total solar eclipse in China on July 29, and summer and fall star parties. Mike also offered some creative IYA ideas. If you want to see the big bang, tune into static on your old TV. And thank those dead stars because without them, you would not be here.

The Canada IYA web site is http://www.astronomy2009.ca or in French http://www.astronomie2009.ca

January Observing Challenges:

Deep Sky: Rolf's pick - Observe the faint nebulae in central Orion. NGC2024 "The Flame Nebula", is just to the left of Zeta Orionis, the leftmost star in Orion's belt NGC2023 below this, is a much dimmer reflection nebula, and will require a larger scope and darker skies. Most difficult is Barnard 33, the Horsehead Nebula

Lunar: Brian's pick The Isabel Williamson Lunar Observing Program (Challenge No.18) is to identify eastern lunar features Palus Somni and Crater Proclus and sketch the ray system extending from Proclus

Thanks to Ann and Art Fraser for the after meeting refreshments.

Viewing The Gateways To The Southern Skies

by Michael A. Earl

The Canadian night sky is rich in stars and deep sky treasures that I have viewed countless times. However, I have seldom viewed the stars lying below the -45 degree declination “boundary”. In January, I had the chance to visit good friends near Orlando, Florida and decided that I would attempt to view a few stars that I had never seen before. Among them were Achernar (Alpha Eridani), Canopus (Alpha Carinae), and the three northern stars of the Southern Cross.

Achernar marks the southern end of Eridanus the river. A very unfortunate aspect of this star is that it lies at a declination of -57 degrees, well below our local horizon here in Ottawa. Those who have tried to trace out the entire Eridanus river have most likely come to the nasty point where the river ends not at Achernar, but at the horizon; the final few stars (including Achernar) hidden just below it. The brightest star of this constellation that most of us have seen is at the northern end of the River; Cursa (Beta Eridani). Cursa lies just off the right foot of Orion at Rigel.

The only solutions to see the river’s southern end are to either travel south or be launched into outer space; the former of the two being the easiest method by far. Since Orlando lies near the Kennedy Space Center, the probability of doing the latter was a little higher! Unfortunately, there were no launches scheduled for the first two weeks of January.

From Orlando’s latitude, Achernar lies at an elevation of only 4 degrees at culmination, so it is possible to view it, but an unobstructed southern sky is necessary.

I spotted Achernar just after sunset on January 2nd. It was bathed in the lights of downtown Orlando, but I still stared at this strange unfamiliar star for some time. Here, revealed to me, was a star that was hidden from me when I was first learning the constellations. I managed to take my first ever images of the complete Eridanus River; from Cursa to Achernar.

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Next up: Canopus. The constellation Carina was once part of a much larger constellation called Argo Navis (Great Ship Argo). In 1922, when the International Astronomical Union (IAU) decided on the final constellations and their boundaries, Argo Navis was split into four smaller constellations. These were Puppis (the stern), Pyxis (the compass), Vela (the sail) and of course Carina (the keel). Most of Puppis and Pyxis can be seen from southern Ontario, but appear very low in the southern sky. Carina and Vela are both hidden from Canada’s skies.

My first ever glimpse of Canopus was at 11 p.m. on January 2nd of the New Year. It appeared as a bright white twinkling star about 8 degrees above the southern horizon. Its brightness appeared about the same as Rigel’s. I attempted to trace out some of Great Ship using Puppis as the starting (and most familiar) point. Even from Florida, the entire ship cannot be seen, but at I could see more of it than I could in Canada!

It was a very strange sensation seeing a bright star below Canis Major. As we know, Sirius appears to be the lowest bright star of the winter southern sky. You begin to believe that nothing else exists below Sirius, yet Canopus is there, waiting for you to travel south to see it.

I have to admit, Canopus did give me some feeling of disorientation. Seeing a bright star so far south in the winter reminded me of Sirius rising, but all I had to do was look higher in the sky to see Sirius with the rest of Canis Major.

From Orlando, Canis Major sits at an impressive 45 degrees above the local horizon! Orion is riding high at 60 degrees!

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I still find it eerie to see both Orion and Canis Major amongst palm trees. In Canada, those two constellations are mostly envisioned amongst leafless trees and snow. The grand finale would occur before dawn on January 5th: the Southern Cross!

My friends and I went out to Cocoa Beach and set up our lawn chairs to see if we could spot the Southern Cross, which was only several degrees above the southern horizon. It did not take me long for me to spot a star that was a good candidate for Gacrux (Gamma Crucis), the top of the cross. It was at the right elevation (4 degrees) and a quick look through binoculars confirmed that the star patterns were indeed correct! I had finally seen the Southern Cross! Mimosa (Beta Crucis) was supposed to have been brighter than Gacrux, but due to its low elevation (only 1.5 degrees) and the high humidity, it looked fainter. Delta Crucis, the faintest of the four, could not be seen with the unaided eye due to the high dewing that morning. However, binoculars picked up the three stars easily.

While I was later reviewing the images of the Southern Cross I had taken that morning, I spotted a fuzzy blob in one of them. I thought to myself, “Could it be?” I knew that Centaurus was just north of the Southern Cross and of course this also meant Omega Centauri (NGC 5139). A quick check verified that Omega Centauri had also been captured! I did not bring my telescopes to Orlando, so I could not get better images. Maybe, next time. Alpha and Beta Centauri were too low to see from Cocoa Beach. Hey, you can’t have everything!

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I was able to view stars down to -60 degrees in declination, an extra 15 degrees into the remaining celestial sphere.

In astronomy, discovery does not always come from a higher difficulty, but simply going to an unfamiliar area and starting over. In this case, I learned a few more constellations in a region of sky I had never seen before. Next time, I will cross this “gateway” and experience the entire southern skies! I thought to myself, “What better way to begin the International Year of Astronomy than learning new constellations?”

Seasons

by Richard P. Taylor RASC Ottawa Centre and PAS Manila

This is the first article in what I hope will be a yearlong series of articles to help celebrate the International Year of Astronomy. These articles will be published (editors willing) both in Astronotes (the monthly newsletter of the Royal Astronomical Society of Canada, Ottawa Centre) and Appulse (the monthly newsletter of the Philippine Astronomical Society.) In the summer of 2008, I returned to Ottawa from a two year stay in Manila. These articles will be based on my personal experiences as an amateur astronomer and as a teacher in both countries.

Now that I am experiencing winter again, after a two and a half year break, seasons are on my mind. I have also been trying to teach some students in their first year of high school the reasons for the seasons. Their misunderstandings and questions have really brought home to me how little most people understand about why we have seasons and the simple fact that different parts of the Earth experience different seasons.

The hardest misconception to overcome is the common belief that it is colder in winter because the Earth is farther from the sun. It is true that the Earth is in an elliptical orbit around the sun, and that it is closer at some times of the year than others. However, the perihelion distance (closest to the sun) is 96.7% of the aphelion distance (farthest from the sun). The intensity of sunlight at aphelion, which is proportional to the inverse square of the distance, is 93.4% of the intensity of sunlight at perihelion. This change of 6.6% has only a minor effect on our temperature. Astronomers are sure that this is a minor effect, because perihelion occurs on January 4.

We are closest to the sun at the time when the Northern Hemisphere is experiencing some of its coldest temperatures. Of course the other strong argument against this "colder when farther away" idea is that different places on Earth experience their coldest season at different times of the year. In Ottawa it's coldest in February, in the Philippines it's coldest in December, in Australia it's coldest in July! If the elliptical orbit caused the seasons, everywhere on Earth would experience the hottest and the coldest temperatures at the same times of year. We don't.

The real reason that we have the seasons is that the Earth's axis of rotation is tilted by 23.5 degrees relative to the plane of the Earth's orbit. Unfortunately, when this is demonstrated to people using a globe and a lightbulb, many of them jump to the conclusion that this tilt causes the Northern Hemisphere to be farther away from the Sun than the Southern Hemisphere during our winter and it is this extra distance that causes us to get colder. Wrong again. This time, it's mainly a problem with our usual models and diagrams of the solar system. The distance from the Earth to the sun is so enormous compared to the size of the Earth that we usually distort the scale to make everything fit. If you could see it properly, it would be obvious that the variation in distance caused by the tilt of the Earth's axis is completely insignificant compared to the distance from the Earth to the sun. The maximum variation of the North Pole's distance to the sun caused by the tilt is only 0.0034% of the usual distance to the sun.

Very long shadows at noon in Ottawa, 2008-12-20
So what IS causing the seasons? Well, there are two effects of the tilt of the axis that are important: the angle of the sun's rays, and the length of day and night. It should be fairly obvious that the angle of the sun's rays has a strong effect on temperature – it happens every day. When the sun rises, it appears low in the sky (near the horizon), its rays are spread out, shadows are long, and the amount of heat we feel is small. At mid-day, the sun is high in the sky (near the zenith), its rays are more concentrated, shadows are small and the amount of heat we feel is greater. As the sun sets, its gets colder again. In Canada, we notice a wide variation in how high the sun appears in the sky at different times of the year. In summer, the noon sun is quite high; about 70 degrees above the horizon or 20 degrees from the zenith. In winter, the noon sun is only about 20 degrees from the horizon.
Icicles hanging from my roof, 2008-12-13
In the Philippines, the variation is much less because the sun appears to move right across the zenith and will sometimes be in the northern sky and sometimes in the southern sky. Twice a year the noon sun will be directly overhead, the rays will be very concentrated and there will be no shadows of vertical objects. Canadians visiting the tropics find this quite amazing. In December, the noon sun will be about 50 degrees above the southern horizon and in June the noon sun will be about 80 degrees above the northern horizon. So it's always pretty warm in the Philippines. Since the angle of the sun's rays has a strong effect on their heat-giving powers, a surface that is tilted towards the sun should get warmer. Quite true. Vineyards are often located on hillsides facing the sun so that they can be warmed more effectively. Just this afternoon, I noticed another effect: icicles! My roof is tilted quite steeply towards the south, facing the low winter sun. The air temperature today never went above minus 10 degrees, but the roof got hot enough to melt the snow. The resulting water flowed down the warm roof until it started dripping off the edge, where it suddenly encountered the freezing air. Instant ice. Growing into beautiful icicles as more water flowed off the roof and onto the hanging ice.

The second effect of the tilted axis is that the length of day and night change throughout the year. Once again, Canadians notice a wider variation than Filipinos. At the moment (mid-December), our sun is rising at 7:34 am and setting at 4:21 pm. That makes for a very short time to get heat from the sun, and as described above, the sun never gets very high, anyway. We are losing more heat at night than we gain during the day, so it keeps getting colder and colder. The shortest day of the year is around December 21 (it changes a bit because of leap years), but this is not the coldest day of the year, it's simply the day we lose the most heat. For another month or so, we continue to lose more heat than we gain each day and it continues to get colder.

Some Canadians live farther north than Ottawa. My sister lives near Saskatoon, Saskatchewan. Her sun didn't rise until 9:08 this morning. In Yellowknife, in the Northwest Territories, it didn't rise until 9:59 am (their highest temperature today was -28 degrees.) In Pond Inlet on Baffin Island, the sun never rose at all. Above the Arctic Circle (23.5 degrees latitude from the North Pole), there are times when they are on the shaded side of the Earth for 24 hours a day. In Manila, I did notice some changes in the length of the day, but they were fairly small by comparison. At the high school where I taught, the school day was shortened by an hour during December so that students could get home before sunset (though I suspect many students and teachers went Christmas shopping instead of going directly home.) But generally the sun rose around 6 am and set around 6 pm all year round. I found this rather upsetting because the temperatures made it feel like summer, but we never had the lovely long summer evenings that Canadians enjoy. In the Canadian summer, the sun doesn't set until 9:30 pm and we have a lovely long evening for outdoor barbeques, swimming or sports. When I described this to one of my Filipino students, he exclaimed "But how can you have any night life, if it doesn't get dark until so late?!"

All night on a basketball court, 2008-04-21
Canadian astronomers are, of course, completely frustrated by these variations in the seasons. When we have the lovely long evenings of darkness, it's too cold to be comfortable outside with our telescopes. When the temperatures get up to the comfort level, the nights become so short that evening twilight is barely finished before morning twilight begins. In this respect, the Philippines have a major advantage. With a reasonable level of clothing, almost any night can be comfortable to stay outside and the nights are always quite long.

!"#$%&'()*&#$'+$,#$-'$.'&&#$/012#341+5$10$6'01&'$'07, the shadow of the flag at the base of the flagpole – 2008-04-29

One of the best nights of observing I ever had was in April 2007 with the PAS. We sat on deck chairs and spread our sleeping bags on a basketball court and watched the stars and planets all night long without ever getting cold. A crescent moon set about 7:30 pm and then until about 5:30 the next morning we had a beautiful, clear, dark sky. I was particularly interested to observe the stars in the southern sky, because I was seeing many of them for the first time. But that's a story I'll save until next month.

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