Star date: 03:15:99

A Clockwork Universe

Long before the advent of digital watches, chronometers, or even the sundial, our ancient ancestors told the time by the stars. It may not be quite as accurate, but time can be told from the positions of the stars, planets, and the Moon on any clear night.

If you can see the Moon,then you do not need any more information. However, if you are using a planet or star to gauge your time, then you will also need to know your location.

To tell the time using the Moon, the secret is to picture the Earth/Moon/Sun system, with the Moon (assuming it is visible) on the opposite side of the Earth from the Sun. With a little imagination, we can picture how as the moon rotates, we view it as different phases throughout it's orbit. If we can see just the "right" side of the Moon, then it is waxing, meaning more of it will be visible the next night. Conversely, if we see just the "left" side of the Moon, then it is waning, and will appear smaller the next night.

When the Moon is full, then we know that the Sun, Earth, and the Moon are in a nearly straight line with one another. So what does this tell us about time? Easy. We cannot see through the Earth, and thus cannot see the Moon at all times. When can we see it?

Imagine the Earth as a tennis ball, and lay a foot ruler on it, resting at the 6 inch mark. This is how much horizon can be seen at any given time. Imagine this tennis ball/ruler system rotating about it's axis, with a golf ball near it, representing the Moon. If the Moon is full (i.e., on the far side of the Earth), we can see how the Moon will not become visible until 9 p.m. local time. It will then be due south at midnight, and will set at 3 a.m.

When figuring out times, use standard times first, and add one hour to the calculated time at the end if you need your answer in daylight savings time. Therefore, if the Moon is full, and just rising, it must be 9 p.m. standard time! If the Moon is first quarter (only the "right" half visible), and due south, it must be 6 pm local standard time. A third quarter moon (only the "left" side visible) just setting would indicate a time of 9 a.m., assuming you could see it!

From this information, we can see how any moon visible just before evening has to be in the east, and be a first quarter moon, or close to it. By the same logic, any moon visible in the early morning hours has to be in the west, and be in third quarter phase, or again, close to it.

If the Moon is not visible, you can still deduce your local time from nearly any other object in the sky, but for this, you will need to know your position, or the rising and setting times of at least one celestial object, coordinated to the nearest time zone to where you are. This is due to the fact that there is no easy way of judging the position of the object relative to the Earth. For instance, Sirius is now bright in our skies in the southeast early in the evening. We can easily look up that for the Keene, New Hampshire area, Sirius is now due south at 6:30 p.m. Given the fact that the Earth turns at 15 degrees per hour (360 degrees / 24 hours = 15 deg/hr), we can see how, if we knew we were 5 degrees to the west of Keene (or any area your rising and setting times are calibrated to), and Sirius was due south, it would have to be 6:50 (5 deg/ 15 deg/hr = 1/3 hr or 20 minutes, and since Earth turns west to east, we add the times, giving us a time of 6:50).

Be careful to make sure your rising and setting times are not more than 1 time zone off, since the human convention of keeping clocks set one hour apart for every 15 degrees will present problems.

Clear skies, and good viewing.

"Understanding is joyous" - Carl Sagan

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