The Apparent Motion of the Sun

Applet

If you prefer to view the applets offline, you can download them. Just unzip astro-applets.zip, and open astro-applets.html in your browser.

If you can't run this applet properly, please download the latest version of the Java Runtime Environment (JRE) at http://java.sun.com/j2se/.

This applet was made together with Tey Meng Khoon and Frederick H. Willeboordse of CITA (Centre for Information Technology and Applications) for the course Heavenly Mathematics: Cultural Astronomy.

Video Clip

I have recorded a video clip that explains this applet. It's available in several formats.

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  RealVideo SureStream, click on the embedded player or click on the link to view in a separate window. The quality and file size of the clip is determined by the preference settings in your RealPlayer.
• RealVideo LAN/150Kbps, click on the link to view in a separate window or right-click to download and view it from your hard disk. This file is big, so this is only for people with fast connections.
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  QuickTime, click on the picture to view in a separate window or right-click the link to download and view it from your hard disk. This file is big, so this is only for people with fast connections.

If you can't get any sound in the QuickTime file, you may need to download the latest version of QuickTime.

Discussion

This Java applet shows the apparent motion of the Sun around the Earth. In other words, we use the Earth as our point of reference.

• What I want to you understand here is how the Sun moves in a spiral of circles up and down. In the course of one day, the declination does not change much, so the path of the sun is almost a circle, but in the course of the year, the declination changes.
• Can you tell when the equinoxes and solstices occur?
• Do you understand why the declination changes?
• Change the viewing angle to 90, so that you're looking straight down at the North Pole. What can you say about the length of the day at the pole?
• Do you understand the relationship between this applet and the one that shows the motion of the Earth around the Sun?