Up in the Air

Another solar bright spot for your calendar

June 18, 2012 

Dr. Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: catonb@appstate.edu.

With the recent transit of Venus and solar eclipse (the former successfully glimpsed in the Carolinas; the latter unavailable east of the Mississippi), we have turned our carefully filtered eyes to the sun a good deal lately.

On Wednesday, our sun reaches its highest point in the sky – the summer solstice. Of course, the sun itself is not going anywhere: The Earth is doing all the moving, going around the sun but keeping its tilted daily rotation axis pointed the same way with respect to the stars. This changes the “rotisserie” alignment throughout the year, from high direct heat now to low heat in winter.

The fact that we are moving and not the sun is not so obvious and was not easy to prove just a couple of centuries ago. One of the earliest clues was stellar parallax: If we are the one moving, then nearby stars should shift back and forth with respect to more distant ones during the year. (Hold your finger up in front of you and look at it successively with one eye open at a time, and you will see your finger against different backgrounds.)

It is odd that some people are unaware of the actual heliocentric system. A Gallup poll showed that about 18 percent of Americans think the sun goes around us. (Polls show similar results in Germany and Great Britain.)

The summer solstice is not the day of the earliest sunrise or latest sunset. The complicated geometry of our slightly elliptical orbit puts those dates a week or so earlier and later, respectively, than the solstice.

Marking the beginning of summer, the solstice reminds us that it is this high, direct sun and the longer day that gives us this season. It is not the slight variation in our distance from the sun, which we are closest to in January.

Finally, translating this motion to the night sky, since a full moon is opposite the sun on the celestial sphere, we find the moon lower in the sky when the sun is higher. You can see this as the moon heads a bit south each night from the new moon (aligned with the sun), the evening of June 19, to its full phase on July 3.

Dr. Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: catondb@appstate.edu. More on this month’s column: www.upintheair.info.

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