Moon phases

A new moon happens when the moon passes between the earth and sun. Usually, the three bodies won't form a perfectly straight line. Rather, the moon passes a little north or south of the path the sun travels along. At first quarter, the moon, earth, and sun form a right angle with the western half of the moon's disk in sunlight. A full moon happens when the moon is on the opposite side of the sky as the sun. As with the new moon, the moon, earth, and sun don't usually form a perfectly straight line. Rather, the moon passes a little north or south of the point in the sky opposite the sun. Last quarter is when the eastern half of the moon's disk is lit up. The moon, earth, and sun again for a right angle.

One full cycle of lunar phases is called a lunation. A widely accepted count of lunation cycles began with the new moon of January 17, 1923. That was the start of astronomer Ernest William Brown's lunation number 1. There have been more than 1,270 lunation cycles since. The Brown lunation numbers are given below. There are other lunation cycle counts, however. Some were started for scientific purposes. Others were started for religious reasons. Astronomical computing expert Jean Meeus proposed referring to the new moon of January 6, 2000 as lunation 0. Computer scientist Herman Goldstine's lunation 0 begins on January 11, 1001 BC. The Islamic calendar's lunation 1 started on July 16, 622. And the Hebrew calendar lunation 1 began on September 7, 3761 BC.

Most people aren't concerned with counting lunar cycles except for curiosity. However, knowing how many such cycles have passed during a given time can be important in scientific discussions about the moon's orbit and in properly rendering religious calendars. To convert the Brown lunation number below to the Meeus number, subtract 953. To obtain the Goldstine number, add 36,152. For the Islamic number, add 16,085. And for the Hebrew number, add 70,281.

The average length of a lunar cycle is 29 days, 12 hours, 44 minutes. The time between successive new moons or full moons will vary by a few minutes because of the gravitational tug the sun, moon, and earth have on each other. While one is inclined to think of this time as the length of the moon's orbit around the earth, the moon actually orbits in just 27 days, 12 hours. The moon completes an orbit when it returns to nearly the same position among the distant stars behind it from our view. But during that time, the earth moves. To get to the same position compared to the relatively close sun takes two more days.

Here are the dates and times in Universal Time for moon phases in the years 2020 through 2039.