Solstice ☉ Equinox ☉ Analemma

Two outstanding long-term photographic projects have come to my attention. Both involved imaging the Sun. Both were published on the popular web site Astronomy Picture of the Day (APoD). The site has presented an astronomy related picture for each day between June 1995 and today. Follow this link if you want to search for a special date. APoD deserves your bookmark. The text below each image on APoD explains how the phenomenon was imaged.

There is a quiz at the end of this post. Don’t give away the correct answer if you leave a comment below. Thank you.

The Analemma

The first image is by German photographer Robert Polzl. Click the image to see the APoD page. What looks a little like a figure eight is an analemma. It is not a single exposure. Instead, it is a composite of 37 exposures, one for each position of the Sun. Here is a link by the photographer to the actual dates of each. The camera needs to be in the same position and at the same time of day for each image. If you had an unobstructed view straight south, you could set the time to be noon for each exposure about every 10 days. Of course, some days will be cloudy. So adjust forward or backward. It won’t matter. That just leaves larger or smaller distances between positions of the Sun. Ignore daylight savings time changes. That would shift the positions of the Sun 15˚ horizontally.

Robert Polzl

The top of the analemma is the summer solstice in late June. The bottom is the winter solstice in late December. The equinoxes are halfway between, not at the intersection. The changes in the vertical position of the Sun is due the the tilt of the Earth on its axis of 23.5˚. During summer, the northern hemisphere tips toward the Sun putting it high in the sky.

There are two reasons why it is a figure eight and not simply a vertical line. Reason one: the Earth is not in a circular orbit. Earth orbits faster during the part of its orbit and slower when it is on the opposite side of the orbit. This faster or slower Earth speed causes the Sun to be a few minutes behind or ahead of the line directly south at noon. Reason two: the axis of Earth is tilted with respect to the plane of our orbit. It is harder to explain that effect and deserves much more than I want to present here. More on that aspect can be found here if you want the details. Both effects are regular and cyclical and cause variation on the time of passage of the Sun. The two variations combine to place the Sun behind or ahead as it transits and forms the figure eight.

Solstices and Equinox

This second image is by Turkish photographer Tunc Tezel. It also appeared in APoD. Click the image to see the APoD page. It is another case where the camera needs to be in the same position. But, in this case the exposures were made on three different dates. On each of those dates, an exposure was made each hour of the day. They were then all superimposed into the composite. The bottom row was taken during winter solstice in December 2007. The middle row during the spring equinox in March 2008. The top row was taken during the summer solstice in June 2008. The rows of images are at different elevations in the sky due to the tilt of the Earth on its axis as it rounds the Sun.

Tunc Tezel

Here is an animation to play with where you have control over the tilt of the axis and revolution around the Sun. You can watch the resulting changes. The tilt of the axis can be set between 0˚ and 90˚. Earth is actually tilted 23.5˚. Speed up or slow down the animation if needed.

Now For Your Quiz

It was stated above that Earth orbits faster during part of its orbit and slower in another part. The orbit of the Earth is an ellipse not centered on the Sun. Orbital speed varies between 30.287 and 29.291 km/sec. For the metrically challenged, that is 67,750 and 65,522 mph, or 18.819 and 18.200 miles/sec. Did you know you were going that fast in orbit? Which situation correctly describes the motion of the Earth in orbit around the Sun? Choose your response in the form below. Assume northern hemisphere observations.

Please don’t divulge the answer if you comment below. Thanks.

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