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The Seasonal Rotation of the Sky (A Reader's Question)     
 
 
 

We got a very nice email recently from Jean in Wisconsin. Jean and her family have shown a lot of enthusiasm for learning the sky, and they are really taking the time to observe and understand the celestial clockwork, as we can read from her note. Jean writes:

" I'll share with you what I have been noticing lately--you are welcome to use any of this in your newsletter--none of this is particularly new to those who have studied the night sky (I am just a newbie in this field!) I have not gotten out for a good look at the sky lately, but have noticed that Orion, which was rising at sunset earlier this winter, is now high in the sky in the early evening. I suppose we will soon have to say good-bye to him and will not be able to find his comforting "belt" until summer has passed. I plan to get my compass out to see if the sun really is rising and setting due east and west now that the equinox is near. I never stop being awed by the deep sky into which we are allowed to observe so few hours out of each day--somehow I am always disappointed if, after waiting for the skies to darken, I find that a cloud cover has come in and that I will not be able to greet my old friends hanging above us.

"I have noticed that the Big Dipper and the North Star seem to be located further to the east than it did in December. Am I imagining it, or is this part of the earth's tilting as the seasons change? I have read that the Big Dipper acts as a 24 hour clock--draw a line through the two stars at the end of the "cup" and this line will travel around Polaris each day. It also acts as a calendar--in early winter it was under the North Star, and now, at the same time of night, it is high in the sky east of the North Star. It will continue to move slowly around its "anchor" throughout the year and every March we will find it back in its "March position". The perfection of our universe!"

Jean, you have made some excellent observations of the sky. I've been avidly following the starry sky since 1988 and the annual progression of the seasons never ceases to amaze me. Here's what's happening....

All of us public school graduates were taught the astro-fact that "the Earth moves around the Sun every 365 days." That fact and a quarter might still get you the morning paper! We can't observe the Earth's motion directly. But as the Earth circles the Sun, the Sun would appear to change position against the background stars behind the Sun. But of course we can't see the stars behind the Sun, due to the obvious fact that the Sun is so bright.

During a total eclipse of the Sun, the sky darkens so that the brightest stars are visible, and the Sun's position can then be seen among the constellations. But such eclipses are rare and not visible from every location. However, after sunset, the constellations visible above the western horizon are the stars to the East of the Sun's position. And before sunrise, the constellations visible above the eastern horizon are those to the West of the Sun. So if we know the constellations, we can conclude that the stars we can't currently see are the ones in the general direction of the Sun.

So anyway, as the Earth circles the Sun, it appears that the Sun is moving toward the East. This motion of the Sun makes it seem that the constellations are steadily creeping toward the sunset over the weeks and months. Over the course of the year, different constellations are visible in the evening sky for each season. Likewise with the Big Dipper. Over the span of seasons, the Dipper also follows the westward drift of the stars. These motions are alluded to in this famous verse from the book of Job:

"Canst thou bring forth Mazzaroth in his season? or canst thou guide Arcturus with his sons?" -- Job 38:32

Many sources including my concordance state that "Mazzaroth" is the Hebrew word for "zodiac," the path of the Sun and planets through the constellations. And the concordance also state that the word translated as "Arcturus" is the "Great Bear," also known as the Big Dipper. So this verse actually points to the seasonal variation in the constellations. So anyway Jean, as you have noticed Orion and the Big Dipper creep forward over the months, you are indirectly observing the effects of the Earth's rotation around the Sun!

In the Fall, the Sun is passing in front the constellations Libra and Scorpius, opposite Orion and Sirius. Therefore, Orion rises after the Sun sets. And now in the Spring, the Sun is passing through the stars of Pisces, so Orion and Sirius are high in the sky in the early evening. In the coming months, the Sun will advance toward Orion's neighborhood, through the stars of Taurus, and Orion and Sirius will eventually disappear into the sunset.

By the June Solstice, the Sun will be in Gemini, and Orion will be invisible. But as the season progresses, the Sun will move off into the constellation Leo. By August, Orion will be visible in the early mornings, rising in the East before the sunrise. Most of the stars have an interval of time each year when they are lost in the glare of the Sun. And in the case of Sirius, this annual disappearance was historically significant....

Read "Sirius and the Solar Calendar."


For more information about topics from Classical Astronomy, please check out Signs & Seasons, a homeschool astronomy curriculum!


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