The Way the Moon Travels

Although we don't recommend traveling at night, many folks do. By using a few rules, the moon can be used to tell direction and the time of night in all but a few nights per month.

The moon produces no light itself; it shines by reflection of the light of the sun. Thus, the illuminated surface of the moon will always point toward the sun, and will show the sun's position even when it has set. An imaginary line through the horns of the crescent moon will always lie more or less north and south. A perpendicular line on the midpoint of this line will point through the illuminated side to the sun's position, whether it is above or below the horizon.

The full moon is always directly opposite the sun: it rises when the sun sets, reaches its highest point at midnight, and sets as the sun rises the next morning.

A waxing half-moon, called "first quarter", rises when the sun is at its highest peak, at noon. The first quarter moon will reach its highest point at sunset.

A waning half-moon, called "last quarter", rises at midnight and reaches its highest point at sunrise.

The "age of the moon" corresponds directly to the shape and size of its illuminated or shining area. The moon takes a little over seven days to grow from the slimmest hairline crescent to a perfect half-moon; a little over seven more days and it is completely full; it another seven days and a bit it has waned into a waning half-moon; and at the end of the full cycle of about twenty-nine and a half days it has completed the cycle by diminishing into a new moon again. To visualize this in a table:

The moon rises and sets an average of about fifty minutes later every night. With one more bit of information, we can determine the age of the moon on any night, and therefore predict the amount of light that will be available from it.

The age of the moon on January 1, 2004 was 8 days (this is the one bit of information). For each year past 2004, add 11. For each month past January add 1 day, then add in the day of the current month. If the result is over 30, subtract 30.

As an example, suppose we want to find the age of the moon on April 24, 2004. We use the following calculations:

The moon on April 24, 2004 is 5 days old.

Knowing now that the moon is only 5 days old, 2 days before first quarter, we can't expect much light from it.

Can we now determine at what time of night this moon will be directly overhead?

We know that a first quarter moon rises when the sun is at its highest and will be overhead when the sun sets. Since the moon rises an average of 50 minutes later (or earlier, depending on the direction you are going) each day, we can determine that a 5 day old moon will rise 1 hour and 40 minutes (100 minutes for 2 days) before the sun reaches its highest point, so it will reach its highest point 100 minutes before the sun sets, which means it will be about 4:20 PM.

Using this same scheme, we can use the relative position of the moon to determine the approximate time of day. Using our example above, if the moon is half way between peak and setting, what time will it be?

Yes, that was too easy! Time between peak and setting is 6 hours, so it has moved half that distance to be between the two points so you just add three hours to the previous answer and you get 7:20 PM. You also know that it will set around 10:20 PM, leaving much of the night in darkness.

Questions: