Wednesday, November 19, 2003

Saros & Sothis:

The Moon was again looking extraordinarily three-dimensional this morning, bulging out over the slowly brightening Eastern skyline. Having the lunar eclipse during last full moon on the 9th makes it likely that there is a solar eclipse this weekend, but we may not be facing in the right direction when it occurs.

Sure enough, we are not. Not even close. As you can see, the lunar shadow will gently brush over Earth's powdered bottom (and no, that's not a reference to Michael Jackson).

Those who were lucky enough to see the lunar eclipse of November 9 may have noticed that the Moon never quite reached a completely even shade of red across its face. Here's a blurry photo I took fairly close to the moment of greatest eclipse:

The moon was never evenly lit because the center of Earth's shadow passed above the Moon, as seen here:

Why is the Moon red during an eclipse? you ask. You don't? Well, let me tell you anyway: because the sky is blue. Lunar eclipses and the sky both have the colours they do for the same reason.

I realized this after someone asked me the "why is the sky blue" question, and I flippantly answered "because all the red is folded into the sunset and sunrise around the edge of the Earth." After a moment's reflection I realized that it was actually true.

The crescent Moon also reminded me that we are approaching the Eid al-Fitr, the end of Ramadan and fasting. The Islamic (Hijri) calendar is not fixed, like the Gregorian. The precise beginning of each month depends on a human physically sighting the first lunar crescent after a full moon. These two factors (human sighting and first crescent), make for a curious situation - it is very possible for different parts of the muslim world to be using different dates on the same day, since the Moon is not visible at the same time from everywhere, and local weather will influence 'first sighting.' If you had really bad weather, the month could stretch out for weeks! Attempts to standardize the calendar have been made many times, but never to full satisfaction. You can see some of that debate here.

Another widely unknown fact about the Islamic calendar is that it is only 354 days long (12 lunar months) - therefore it slowly shifts with respect to the Christian calendar. What is obvious is that a great deal of scientific effort goes into calculating the probable calendars for the Islamic world. Why then, having such extraordinary mathematical talent so early in mankind's history, did they settle on this system? Because that was the interpretation of the Koran: Sura 9 verses 36-37, and Sura 2 verse 189. There are 12 months. They are determined by the Moon. No more, no less. And that was that.

There is a great discussion of calendars here, where I also learned that even though the month and day counts have hiccupped several times as we adjusted the calendar (e.g. 2 September 1752 was followed by 14 September 1752 for the Julian to Gregorian switch in the United Kindgom and colonies), the seven day cycle of the week (Monday, Tuesday, ... etc.) has not been broken since at least 1400 BC, and possibly far before that.