Archive for March 22, 2021

03/22/2021 – Ephemeris – The Moon and the week

March 22, 2021 Comments off

This is Bob Moler with Ephemeris for Monday, March 22nd. Today the Sun will be up for 12 hours and 16 minutes, setting at 7:58, and it will rise tomorrow at 7:39. The Moon, 1 day past first quarter, will set at 5:08 tomorrow morning.

The Moon tonight is about as far north in the sky as it gets this month, and being six days before full, and this full Moon is the first full moon of spring, that means Easter is less than two weeks away. Did you notice that a week is nearly equal to a quarter of the Moon’s orbit of the Earth? If you look at a calendar that has the Moon’s phases on it the phases are 7 or 8 days apart. It’s a good thing that no one decided to adjust the length of the week to accommodate the Moon’s phases. It’d be hard to plan your weekend. Our calendar is pretty screwed up as it is with months of 28, 29, 30 and 31 days all set by politicians… Roman politicians at that. I haven’t heard much about calendar reform lately, so I guess we’re stuck with it.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.


I wrote the above before actually checking out the historical origin of the time period we call a week. It turns out that the week is really related to a quarter of a lunation or lunar month. The Babylonians fixed the fact that seven days is a bit less than a quarter of a lunation by adding a day or two to the last week of the month, so the next month’s new moon started on the same day of the week.

Here the new moon is defined as the first appearance of the Moon in the evening sky after disappearing from the morning sky. We currently define the new moon as the Moon’s solar conjunction. Its first appearance in the evening sky would generally be the next day, or rather evening. The numbering still works out. Our new moon is day zero of the lunar month.