Bob Moler's Ephemeris Blog

This is Ephemeris for Tuesday, August 15th. Today the Sun will be up for 14 hours and 3 minutes, setting at 8:48, and it will rise tomorrow at 6:46. The Moon, 1 day before new, will rise at 6:41 tomorrow morning.

Once one finds the Milky Way which is pretty hard to miss this month, it becomes obvious that the Milky Way is split lengthwise starting near Deneb, the northernmost star of the Summer Triangle, almost all the way to the Teapot of Sagittarius low in the south. That dark split is called the Great Rift. Galileo first discovered that the hazy clouds of the Milky Way we’re actually made of faint stars, so it was thought the dark areas were due to a lack of stars. The great 18th century astronomer William Herschel did star counts all over the sky with his telescope. A map drawn of the shape of the Galaxy shows the lack of distant stars in the direction of the Great Rift. It’s not really fewer stars, but interstellar clouds of dust blocking the light of stars behind them.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT –4 hours). They may be different for your location.

The span of the Milky Way across the summer sky as seen in the free Stellarium app. Specifically August 15th at 10 pm from Northern Michigan. The Milky Way has been set to its maximum brightness to show as much detail as possible. It really isn’t that bright. The horizon is only delineated by the compass points. The dark clouds of the Great Rift are also visible. The Milky Way, in Stellarium, is photo-realistic. The triangle of three bright stars above center is the Summer Triangle, Specifically, clockwise from the top star: Vega, Altair and Deneb. Large images can be truncated on the right. Click or tap on the image to enlarge and display the complete image.

William Herschel’s vision of the cross-section of the Milky Way from 1785, drawn by his sister and collaborator, Caroline Herschel. The Great Rift is seen as the gap between the fingers of stars on the right. Herschel thought the rift was caused by a lack of stars. Other than that, he got the flattened shape right, for what he could see within ten thousand light years. The center of the Milky Way lies 27,000 light years to the right. The spiral structure of the Milky Way wasn’t determined until the advent of radio telescopes in the mid-twentieth century.