It is often said that the moon rises, on average, about 50 minutes later each day.
Let’s examine all moon rising times shown in the 2017 table for San Diego created with data from the Astronomical Applications Department of the U.S. Naval Observatory.
On average during the year, each rising time is 24 hours and 50 minutes greater than the previous one.
In non-leap year 2017, there are 365 * 24 = 8,760 hours. Dividing that result by the average time between moon risings (24 + 50/60 hours) gives an expected total of 353 moon risings in a year which is what one finds on the year 2017 table.
Looking at the 352 actual time differences between the successive moon risings, one finds the shortest time of lateness is 34 minutes and the longest is 66 minutes. That is a much wider range of lateness minutes than I had expected.
Below is a graph of the lateness minutes of moon risings in year 2017. The maximum of 66 minutes is circled in red where it occurs on 3 dates. The minimum of 34 minutes is circled in green. The average of 50 is shown with a gold line.
Knowing range allows for a fuller understanding than does average alone.
Recently while looking at a crescent moon at sunset, someone asked me: waxing or waning? To my great chagrin, my answer was wrong.
If I had noticed a new moon on a previous day, then the answer would obviously be waxing. But there’s another way to get the correct answer.
When seen about the time of sunset, the moon is always waxing, and a quarter moon is always 1st quarter. Whatever moon phase that you see about the time of sunset, you can be sure that at the next sunset, the moon will illuminate more area until it gets to full.
This diagram shows the range of possible moon phases at sunset.
When seen about the time of sunrise, the moon is always waning and a quarter moon is always 3rd quarter. Whatever moon phase you see about the time of sunrise, you can be sure that at the next sunrise the moon will illuminate less area until it gets to new.
This shows the range of possible moon phases at sunrise.
At midnight if the moon is in the east (rising) then it is waning; if it is in the west (setting) then it is waxing.
This shows the range of possible moon phases at midnight.
From the admiral's chair
John Berol is the husband of Captain Diane. He commissioned Celtic Song in 2005, has sailed extensively and maintains an active interest in both the boat and her captain. He believes the more you know, the better you will sail. The term “Admiral’s Chair” is a family joke. For just as every writer needs an editor, so every captain needs an admiral.