A Very Low Moon

Did you have a very difficult time locating the moon for קידוש לבנה this past מוצאי שבת? It was extremely low and close to the horizon. I was discussing this with Rabbi Dovid Heber and he provided some very interesting findings, after noting that he doesn't recall ever having such difficulty seeing the moon for קידוש לבנה due to altitude.

First, a little background discussion on "moon mechanics" may be necessary to understand what was going on. I find it somewhat surprising but it seems there are many who are not aware that the moon rises in the east and sets in the west just like the sun. However, the precise location and times fluctuate much more erratically than that of the sun. Let's not worry for now about where it is but more about when it is. 

At the beginning of the month, on the מולד, the moon sits between the earth and the sun. Therefore, it follows that the moon should rise and set at the same time as the sun, approximately. However, as the month goes on, the moon rotates around the earth and so its rise and set times will quickly deviate (later) from the sun's. How much should it deviate? That is relatively simple to calculate. While it spends the entire month rising and setting later and later, when we get back to the beginning of the next month, it's right back where it started. This means that the rise/set times have come around full circle - approximately 24 hours. Divide 24 by the average length of a month, approximately 29½ days and that yields...

If you want to avoid the math, skip here: the moon should rise and set close to 49 minutes later every day of the month. The מולד of אלול was on Tuesday afternoon. Four days later, the moon should be setting close to 3½ hours after sunset. That should leave it sufficiently high in the sky after מעריב to be seen by all. However, observe the moonset times here. (This is for Baltimore but the phenomenon is related to the moon's path and is evident no matter what city you look at.) Somehow, it is only advancing around 20 minutes - or even less - every day. Later in the month (starting tomorrow) the times seem to skew the other way such that everything more or less evens out by the time. So, the moon was still setting much closer to sundown than one would expect that many days in to the month and that is why it was so low. 

Unfortunately, my ability to explain this even ends here. I can only illustrate that this was happening but I don't know exactly why. I have not done a full inspection but I assume if this happened once, it probably happens with some degree of frequency. But I haven't inspected moonset times on a greater scale yet to have any idea what that is. I hope this cleared up some things. And if you weren't able to say קדוש לבנה yet, you do have until... uh oh, there is a lunar eclipse coming up next week. See more