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This is the Classical Astronomy Update, an email newsletter especially
for Christian homeschool families (though everyone is welcome!)
Please feel free to share this with any interested friends.
IN THIS UPDATE:
- Announcements
- Backyard Compass - Now Linked!
- Astronomy Topics
- "Sundogs" - Parahelic Rainbows
- Dance of the Planets
- Venus and Jupiter (Earlybird Special)
The day is thine, the night also is thine: thou hast prepared the light and
the sun. Thou hast set all the borders of the earth: thou hast
made summer and winter. - Psalm 74:16-17
Welcome to the Classical Astronomy Update!
Hello Friends,
Our family joined a local homeschool support group this year and it has been a great experience for our kids. I'm especially excited that the three oldest are in ski club since I've been an avid skier since 1976, as time and money has allowed. One of the great things about parenthood is sharing the things you love with your kids. Homeschooling has given us many such opportunities, and for school credit!
Announcements
Backyard Compass - Now Linked!
I apologize that our last Update did not include a link to the "backyard compass" blog post showing the length of noon shadows on the winter solstice. Please follow this link to see how the lengths of the noon shadows compare on the summer solstice, autumnal equinox, and winter solstice.
As mentioned before, the "backyard compass" is the first field activity in our Signs & Seasons curriculum. Your family can learn how to easily construct one of these and use it to find direction in the day or night for any time of the year, and also make nifty observations such as the seasonal changes in the noon shadows.
Thanks to all who are using Signs & Seasons. If you've considered using this curriculum, keep in mind that winter is an excellent time of year to begin learning the constellations.
Astronomical Topics
"Sundogs" - Parahelic Rainbows
Anyone out there ever heard of a sundog? These are an atmospheric phenomena that happen near the Sun when there is a lot of cirrus cloud in the sky. Even if you've never known the name, odds are you might have seen one and not known what it was. Sundogs are actually very common, especially during the wintertime. Here's a repost of an old, old Update article about sundogs from 1996, back when it was called The Starman Update. It starts with a question from my friend Dr. Parsons in Jackson, Michigan:
Jay-
Here's a question for you (to answer when you have the time) Last week I saw and interesting set of rainbows. It was late afternoon and the sun was about 5-10 degrees above the horizon. It was a dry day and the sky was mostly blue with scattered high altitude wisps of clouds. In the west it was more overcast. Around the sun was a perfect unbroken circle rainbow (it dipped below the horizon from about the 5:00 to 7:00 position). At various places along the arc, breaks in the clouds produced beautiful golden beads, much like you see photographed just before a solar eclipse totality. It was quite a sight. What was most interesting, however, was a second rainbow visible at the same time which was not concentric with the sun or the first rainbow. It appeared much higher in the sky above the first rainbow and it bowed away from the circle rainbow around the sun. It appeared to bow around a central point located 70-80 degrees from the sun toward the zenith. Have you ever heard of anything like this? How would you explain it?
Curious,
Dave
What you saw was a classic textbook example of a parahelic circle with attendant phenomena. It is caused when the Sun is low to the horizon and sunlight passes through high altitude cirrus clouds. Sunlight is refracted through ice crystals in the clouds to create a circle 22 degrees around the sun. The most prominent part of the circle are clumps which fall horizontally to the sun along the circle. These are called "sundogs" or "mock suns" and are often visible, alone or as a pair, even when no circle is produced. Sundogs result from the fact that the ice crystals, being flat hexagonal discs, tend to lie flat in the air, being buoyed up by the atmosphere. From the observers line of sight, the greatest concentration of refracted rays lies horizontal to the sun, and so these are the most prominent part of the cirlce. You and I once saw a single sundog as "a patch of rainbow next to the sun" as we called it at the time. At the time, I had never heard of sundogs, and even though I had seen one before (not yet connecting it to the one that day), that was the best one to that date.
When the Sun is very low, becoming dimmer and turning orange near setting, the sundogs get brighter and also turn orange, rivalling the sun in brightness. This phenomenon was observed in antiquity and was referred to by Pliny the Elder in his "Natural History." In this book, regarded as the first encyclopedia, Pliny discusses legitimate natural phenomena along with griffins, minotaurs and other pseudoscience. His passage, where he says "sometimes there are three suns," is considered to be further evidence of his ignorance by modern "enlightened" critics who are not aware of sundogs.
These halo effects are really common in the polar regions. There is a halo report in the account of the ill-fated expedition of William Barents, whose ship in 1596 was stranded over the winter in arctic Nova Zembla (latitude 76N). The account includes a great sighting of parahelic phenomena:
And when the Sunne was about South-South-eaft, wee faw a sftrange light in the Element: for on each fide of the Sunne there was another Sunne, and two Raine-bowes that paft cleane through the three Sunnes, and then two Raine-bowes more, the one compaffing round about the Sunnes, and the other croffe through the great rundle (circle).
This report describes sundogs along a parahelic circle, with another "raine-bowe" crossing the parahelic circle (probably at the top.) Sounds pretty much like what you saw, huh?
The perfect circle you saw was very rare, especially with the elusive upper tangential arc. I've only seen one of these one time, and it was pretty weak. This past fall (1995), I saw a rare paralunar circle around the moon, complete with the rare "moondogs." However, sundogs are very common and you can find them in all seasons where wispy cirrus clouds are seen 22 degrees on either side of the Sun. Now that you've observed them, keep your eyes open and you'll be surprised how common they are.
There are a couple great books on the subject, one by Fred Schaaf and another
by Minnaert. Check them out at the library.
Dave replied to say:
You described the conditions and what I saw exactly - sundogs and the elusive upper tangential arc. What I was suprised to see and perhaps I didn't describe it well was the separate arc that was 20 degrees or so above the upper tangential arc. This arc curved away from the sun and the primary arc with the sun dogs. It appeared to arc around a point near the zenith, not the sun. Have you ever seen something like this?
There are all many secondary, tertiary and higher order phenomena, which depend crucially on the conditions. There are so many different arcs that occur on all sides of the parahelic circle. What you saw was extremely rare, so don't expect to see many more! Get the books and learn them all.
After that, my friend Jim from Maryland had a followup question:
Your note on parahelic rainbows was interesting. I have a related but less dramatic question for "Ask Starman." That is, while driving after a rain we saw a beautiful rainbow that reached our car and rode with us. Depending on the spray from our car, the rainbow could be meeting our left front tire and then move a car width away or further, then later come back to us and remain for a bit. Rather than contacting the X-Files producers, I thought you might be able to give me the scientific stuff. Yea, I know it was just reflecting off the spray, but I want to know how it was seeming to be miles away in the sky while simultaneously touching down right next to us--it was a continous ribbon that was . . . amazing. It was like something out of an M.C. Escher print.
Jim, rainbows are always a perfect circle, but trees and stuff cut off the bottom half. The rainbows are formed by sunlight refracting and reflecting through raindrops. If the raindrops are far away, the bow seems far away and is cut off by the trees. However, if the drops are close, like all around you, you can see more of the complete circle. The rainbow's not really miles away, but just seems to be. Its an illusion. This past spring, I was in a downpour and the sun was shining. Part of the rainbow was in the foreground in front of the trees, similar to what you saw.
Ever see a rainbow in a garden hose? You can move the hose to see different parts of the circle. I've seen rainbows from airplanes where there were no horizons and you can see the complete circle.
For more info on sundogs, google it or check out this Wikipedia page. Keep on eye out for sundogs this winter. If you see anything good, please reply with a detailed description and we'll include it in an upcoming Update.
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Dance of the Planets
Planetary Conjunction of Venus and Jupiter (Earlybird Special)
If you're up before the sunrise in late January, 2008, be sure to look to the southeast where you will see the bright planets Venus and Jupiter. These bodies are approaching each other and will meet in an uncommonly close conjunction on the morning of Friday, February 1.
For those who have never seen these celestial bodies, Venus is the brightest object in the sky after the Sun and the Moon. It is always seen as either the Morning Star or the Evening Star, staying near the Sun as it swings to the east and west in its orbit.
Jupiter is the next brightest object in the sky after Venus. It's surprising that so few people today realize that the classical planets are not only easily visible but are the brightest "stars" in the sky. At any rate, whenever Venus and Jupiter line up in the sky, it's always a lovely sight.
If you keep your eye on the morning sky over the next week and a half, you'll see Venus and Jupiter drawing close together. But you'll need a clear, flat horizon to the southeast to see this conjunction since these planets are very near the Sun at this time and will be very low to the horizon as the morning twilight brightens.
In this conjunction, Jupiter is emerging from behind the Sun as seen from Earth, and Venus is circling around behind the Sun, and will disappear into the morning twilight sometime in late May or early June. Though Venus is close to the Earth and Jupiter is very far beyond our orbit, these planets just happen to lie along the same line of sight as a result of where they are in their celestial cycles.
Even though these bright planets pass each other on February 1, they will still be close together in the morning sky for several days. On the morning of Monday, February 4, this bright pair will be joined by the old waning crescent Moon. This should be an amazing sight as the three bodies form a neat little triangle just above the horizon. If you're not an earlybird, be sure to set your clock for this one!
For detailed information about planetary and lunar conjunctions, complete with illustrations, check out our Signs & Seasons curriculum.
Coming in future Classical Astronomy Updates:
Til next time, God bless and clear skies!
-jay
The Ryan Family
Cleveland, Ohio, USA
When I consider thy heavens, the work of thy fingers, the moon and
the stars, which thou hast ordained, what is man that thou art
mindful of him? and the son of man, that thou visitest him?
- Psalm 8:3,4, a Psalm of David
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