moons calendar

The Moons Calendar

The Moons Calendar is a lunar calendar which illustrates how the moon's appearance, the moon's position in the sky, and the days of the lunar month are related. Rather than superimposing the moon's appearance on a standard calendar, the Moons Calendar displays each lunar month as a horizontal strip of days stretching from right to left. This strip of days represents the roughly 390 degrees that the moon travels through the lunar month from new moon to new moon. The new moon which initiates the month is drawn at the far right followed by the waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent, and subsequent new moon in order from right to left.

This layout imitates what we see through the lunar month from the northern hemisphere. In the temperate latitudes of the northern hemisphere, the moon spends most of its time to our south, so the calendar is oriented as if you were facing south. The daily motion of the moon is to rise in the east and set in the west, which is a motion from left to right (east to west) when facing south. But the motion of the moon through the month goes in the opposite direction, from right to left (west to east) across the sky. In the southern temperate regions, tropical regions, and polar regions you would see lunar month differently.

To see how the layout works you need to observe the moon for a few days at approximately the same time each day. For instance, if you see the first crescent moon after a new moon at sunset, the moon will be in the west, slightly to the east of the setting sun. On the next evening a larger crescent moon should be visible at sunset. This second crescent moon of the month will be further to the left (east) from the setting sun than it was on the day before. If you continue tracking the moon at sunset, you will see it grow larger and appear further to the left (to the east) each day.

In the Moons Calendar we draw the phases of the moon in the same order that they appear in the sky. The waxing crescent moon is drawn to the left (east) of the new moon, the first quarter moon is drawn to the left (east) of the waxing crescent moon, and so on through the lunar month. Each moon phase symbol is drawn centered on the date and time UTC at which the moon phase occurs. The phases depicted are exact eighths of a lunar period. Midnight UTC is marked either by ticks or by a vertical line, and the vertical lines also mark the beginning of each calendar month. The days of each calendar month are numbered in the lower right corner of the day's square. The year and month of the beginning (end) of each lunar month appear in the right (left) margin,

Adjacent lunar months are aligned by the full moons. The other phases do not align, they form waves. These waves show the variation in the speed of the moon in its orbit. When the moon is closer to the earth, it moves faster. When the earth and moon together are closer to the sun, they both move faster. These variations combine to make the different parts of the lunar month pass more or less quickly.

This calendar is implemented by open source software which can compute and display the calendar in most web browsers. The details are described here. The images on this page are static.