Crescent Supermoon Smiles In The West Tonight

This illustration shows the supermoon crescent looking northwest about 45 minutes to an hour after sunset tonight (May 27). Created with Stellarium

You normally hear the term supermoon applied to an unusually close (and large) full moon, but the truth is, a supermoon can occur during other phases, too. The closest supermoons happen during full and new moon, the two times a month when the sun, Earth and moon are lined up in a row.

The moon’s orbit around Earth is an ellipse with one end closer to the planet (perigee) and the other farther (apogee). A supermoon occurs when the full moon is close to perigee. Not to scale. Credit: Bob King

This month’s supermoon occurred on May 25 at 9:24 p.m. (CDT) just 5 hours after new moon, when Earth and moon were only 221,959 miles (357,209 km) apart. Since the moon’s average distance apart is 239,000 miles, that’s significantly closer — more than two Earth diameters. Problem is you can’t see a new moon except when it passes in front of the sun during a total solar eclipse, when it appears in perfect, black silhouette. But we can come close.

Tonight, look low in the northwestern sky at dusk for a  beautiful (and nearby) crescent moon. If it looks bigger than normal, it really is. Just two days past perigee or closest approach to Earth, the 2-day-old crescent will be just under 1,400 miles further than on the 25th, close enough to still be super.

The sun, totally eclipsed by the new moon, as viewed from a plane on March 20, 2015. The dark area is the moon’s shadow on the clouds. Credit: Reuters/Sergei Karpukhin

Had this month’s close and relatively larger moon eclipsed the sun, we would have experienced a considerably longer totality (when the sun is totally eclipsed) than the one during the upcoming August 21 eclipse. Why? A bigger moon covers the sun for a longer time.

The longest totalities happen when the new moon is closest to Earth at the same time Earth is farthest from the sun. That happens every year in July. A big moon and a smaller-than-normal looking sun combine to make a s-l-o-w eclipse. The longest total solar eclipse during the 11,000 year period from 3000 BC to at least 8000 AD will occur on July 16, 2186, when totality will last 7 minutes, 29 seconds. The longest total eclipse of the 21st century already happened: July 22, 2009 with a 6 minute, 39 second totality.

Careful of those sharp points! Chris Schur photographed the 19.2-hour-old, one-percent illuminated moon yesterday morning.  Credit: Chris Schur

Seeing the youngest (evening) or oldest (morning) crescent moons makes for an exciting challenge. Tonight’s moon will be easy to spot, but on May 26, Chris Schur of Payson, Arizona saw and photographed a 19.2-hour-old moon. Some of us have seen a 24-hour-old luna, but 19 hours is a great feat! Let’s hear Chris tell the story in his own words:

“This shot has been in the planning for at least half a year now, the opportunity came about this morning, with the rising sun halfway up in the sky in the east. This is by far the thinnest crescent moon I have ever imaged. Here the moon is .80 days old — 19.2 hours according to the Virtual Moon Atlas, and the illumination is 1.0%. The moon was just under 15 degrees east of the Sun.

“And yes, I did view the crescent visually, and it was really tough! I could see only a portion of the crescent at one time in a low power ocular, but it was there against a light blue sky. Few features are seen on the limb, many black shadow filled craters are present at this illumination.”

Beautiful photo, Chris!