My new Sky Clock

Planets don’t look like stars. Their light is strong and steady, unlike the pale twinkling of their starry neighbors. Once you know what to look for, it’s easy enough to spot them. But since they wander around relative to the stars (“planet” literally means wanderer), it’s not always easy to tell one from another.

I often find myself looking at the evening sky and saying “I know that’s a planet, but which one is it?” To answer this question, I built the Sky Clock. It shows where the sun, moon, and five visible planets are situated relative to earth. The inner ring is occupied by the sun, the next is the moon, and third ring contains the planets. Outermost are the constellations of the zodiac, the backdrop against which the planets move. In a sense, this is view is a throwback to a pre-Copernican world: it shows the sun going around the earth. But at the same time, this is the true experience we have as humans standing on this planet. Stargazing is profoundly and inescapably geocentric. To a good first approximation, the sun goes around us, not the other way around.

The result is an accurate representation of where the planets can be found along the ecliptic, which is the great circle the sun traces through the sky on its yearly journey. For the accuracy, I am depending on the ephemerides.com site sponsored by JPL. This tells you where in the sky to find each of the planets. Then I use my rudimentary PHP skills to draw the map. Everything in the brown part of the diagram (roughly speaking) is below your feet. The sun is directly beneath you at midnight. Everything in the blue upper part of the diagram is in the sky, though of course anything that shares the sky with sun can’t be seen. As for the letters in the triangles, E and W do stand for east and west respectively, but N stands not for north but for nadir, while Z is for zenith, these being the lowest and highest apparent parts of a stars daily travel.

UPDATE: Shame on me for not testing before shipping. I didn’t realize that the “transparent” backgrounds on my PNG files (the small planet images) were not in the least transparent on Internet Explorer. I developed it on Firefox, and it looked fine there. However I found a dandy JavaScript fix written by Bob Osola that, on my computer at least, makes it look presentable again on IE. Let me know if it still looks broken on your browser.