Eric Bergman-Terrell's Blog

Don't Miss the Lunar Eclipse on Sunday & Monday
September 26, 2015

My Astronomy Lab 2 program has predicted a very nice lunar eclipse, visible on Sunday night (September 27, 2015 8 pm Mountain) into Monday morning. Because the moon will be very close to perigee (point closest to Earth) during the eclipse, it will be a nice one.

Like any lunar eclipse, one can say that "there won't be another lunar eclipse quite like this one for 18 years, 11 days, and 8 hours". This period of time is called a saros and was discovered by ancient Babylonian astronomers. Anytime there is a lunar or solar eclipse, there will almost always be a very similar lunar or solar eclipse, one saros later. It might be a day or two earlier or later, due to leap years. And unfortunately, that eclipse may not be visible from the same place, because it will happen 8 hours later.

There are a few recurring events that are involved in a lunar eclipse:

The moon goes from full to full every synodic month which is approximately 29.53059 days. The moon goes "up" through the ecliptic once a draconic month which is about 27.212220 days. The moon goes from perigee (point closest to Earth) to the next perigee every anomalistic month, which is about 27.55455 days. All of these cyclic phenomena coincide every saros.

All of these cycles coincide every saros, because there are almost exactly 223 synodic months, or 242 draconic months, or 239 anomalistic months per saros:

1 mean Gregorian year = 365.2425 days

1 saros = 18 years, 11 days, 8 hours

           = 18 * 365.2425 + 11 + 8/24 days
           ≈ 6,585.46 days

1 saros / 29.53059 days per synodic month ≈ 223.00 synodic months / saros  

1 saros / 27.21220 days per draconic month ≈ 242.00 draconic months / saros  

1 saros / 27.55455 days per anomalistic month ≈ 239.00 anomalistic months / saros  

Babylonian astronomers discovered the saros several hundred years B.C.E. This puzzles me, given the 8 hour component of the saros. A solar eclipse viewed from a given location will tend not to be visible 8 hours later, because it will probably be night!

But back to this week's eclipse! Astronomy Lab 2 has predicted that the moon will be at perigee on September 27, 2015 at about 8 pm:

Lunar Apsides Report
Lunar Apsides report

... and the total phase of the lunar eclipse happens just around that time. The moon will be big and beautiful during the eclipse.

Be sure to look up and watch it:

Lunar Eclipse Report
Lunar Eclipse Report

Lunar Eclipse Movie
One Frame from a Simulation of the Eclipse

If you're interested in Astronomy, download Astronomy Lab 2, which features reports that predict the most important astronomical events, graphs of interesting astronomical phenomena, and animations of future and past events, exactly as they would appear.

Postscript: The eclipse was big, red, and beautiful over here in Colorado. I hope you got to see it too!

Photo of 2015-09-27 lunar eclipse

Keywords: Lunar Eclipse, Astronomy Lab 2, Perigee, Saros, month, synodic month, draconic month, annomalistic month, Solar Eclipse, astronomy

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