Timing Planetary Light Shows

A post for the Ecotone Wiki topic on time and place.

When you think about it, we don’t need to know the exact time for the vast majority of events. If you have a 10 AM meeting, it’s okay if you enter at 10 o’clock and 47 seconds. Baseball games never begin exactly at 7:05 PM in the evening (make it 7:06 or even 7:11), and movies always have trailers when they begin. Oddly, the class of events that I’ve encountered in the course of my life that requires the most precise time lies not in human affairs, but in nature.

Celestial mechanics is one of those amazing triumphs of science, and the ability to precisely predict where solar system bodies will be when is what makes solar system observational astronomy a lot of fun. Is tonight the night when three of Jupiter’s moons will be eclipsing the giant planet? Time to set up the telescope then. And solar eclipse chasers know exactly when to book their special cruise ship on the other side of the world. But there is a solar system observational game that though a bit more obscure, is quite entertaining in its own right.

I refer to occultations. An occultation occurs when a solar system body moves through the line directly between an observer on Earth and another solar system body or a star. The easiest of these to observe is a lunar occultation. As the moon orbits, it changes its position with respect to the fixed stars. Sometimes it even moves directly in front of a fairly bright star.

It’s an amazing event to watch through a telescope, especially if the side of the moon that eclipses the star is not illuminated at the time. The moon slowly, but as steady as anything, creeps up on the star. The moon gets closer and closer to the star, then all of a sudden, within a tenth of a second or so, the star just winks out.

Not all occultations are as quite as dramatic, and these call for more effort on the part of the observer. Sometimes an asteroid invisible in a small telescope will occult a faint star. What happens then is that the star will dim measurably for a bit, and then return to full brightness.

It turns there’s good amateur science to be done here, because with precise measurements of exactly when these occultations take place, we can improve our knowledge of the orbits of these solar system bodies. The International Occultation Timing Association coordinates the activities of the relatively small band of amateurs interested in this activity. By precise I mean on the order of 0.1 to 0.01 seconds, but this is achievable with a camcorder and a precise time signal.

And where does one get this precise time signal? The easiest way is via shortwave radio. In the U.S., the station WWV broadcasts time signals on frequencies of 2.5, 5, 10, 15, and 20 Mhz 24 hours a day.