What I imagine might be frequently asked questions.
- 390 degrees? Why isn't it 360 degrees?
- How would it work in the southern temperate regions?
- How would it work in the tropical regions?
- How would it work in the polar regions?
- Mostly south?
390 degrees? Why isn't it 360 degrees?
The moon will return to it's original position against the stars after travelling 360 degrees.
But to finish the lunar month, the moon has to to catch up with the sun and make a new moon,
and the sun is also moving east against the background of the stars.
The moon travels 360 degrees in about 27.3 days, or about 13.2 degrees per day. The sun travels 360 degrees in a year, slightly less than a degree per day. On average it takes the moon 29.5 days to catch up with the sun, so it has to travel 389.4 degrees to finish out the lunar month.
The time it takes the moon to go 360 degrees, 27.3 days, is the sidereal period. The time it takes the moon to go from one new moon to the next new moon, 29.5 days, is the synodic period.
How would it work in the southern temperate regions?
In the southern temperate regions, the moon spends most of its time to the north, so we
would observe it facing north. Thus the moon rises to our right (the east) and sets to our left
(the west). So the daily motion of the moon is from right to left (east to west), and the monthly
motion is from left to right (west to east). The calendar would be drawn with the days increasing
from left to right. The new moon which starts a lunar month would be drawn at the far left followed
by the waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning
crescent, and subsequent new moon in order from left to right.
How would it work in the tropical regions?
In the tropics the moon is neither mostly north nor mostly south. The moon is sometimes
north, sometimes south, and sometimes directly overhead. If you lie on your back with your
feet toward the south, then west is on your right and the northern hemisphere calendar would
work.
If you lie with your feet toward the north, then west is on your left and a southern hemisphere
calendar would work better.
How would it work in the polar regions?
North of the arctic circle the sun never rises for part of the winter and never sets for part
of the summer. The moon does essentially the same thing on a monthly basis. For half a sidereal
period, the moon is north of the celestial equator and it does not set. For the other half period,
the moon is south of the celestial equator and it does not rise.
Mostly south?
Viewed from the north temperate regions, the moon is always to the south of overhead when it reaches
its highest point during the day or night, so it makes sense to watch it facing south. When the moon
is north of the celestial equator, it will rise north of east and set north of west
The Moons Calendar is Copyright © 2006,2009 by Roger E Critchlow Jr, Santa Fe, New Mexico, USA. All rights reserved. Permission granted for reproduction for personal or educational use.