October 4
The Great Square Rises
Sun Position
The Sun is in Libra near -5° declination. In the Northern Hemisphere, the shortening of days is now unmistakable in the early-evening darkness; in the Southern Hemisphere, late light is starting to linger.
Sky Highlight
The Great Square of Pegasus (formed by Markab, Scheat, Algenib, and Alpheratz) is nearly overhead for mid-northern latitudes on October evenings, making it one of autumn's most reliable navigation landmarks. It is a useful tool for testing sky darkness: how many stars can you count inside it?
Deep Sky Object
NGC 7331, a spiral galaxy about 40 million light-years away. NGC 7331 in Pegasus is a bright, nearly edge-on spiral galaxy often called the Milky Way's twin (similar in size, structure, and activity) making it a convenient mirror for imagining what our own galaxy might look like from outside. Well-placed for Northern Hemisphere observers in autumn; visible from southern mid-latitudes but lower in the sky.
Featured Star
Algenib (γ Peg) is a blue subgiant (B2IV) about 391 light-years away, one of the four stars that define the Great Square of Pegasus, though it is technically shared with the constellation Andromeda in some older atlases. Its spectral class and distance put it in a very different physical category from its three square-mates. Algenib, the flank of Pegasus, cornerstone of the Great Square.
Around This Date
- October 4, 1957Sputnik 1 launched, becoming the first artificial satellite to orbit Earth and emitting the radio beeps that marked the opening of the space age.
- October 4, 2004SpaceShipOne won the Ansari X Prize by completing its second crewed suborbital spaceflight within two weeks, marking the first privately funded crewed spacecraft to reach space twice.
The Great Square is nearly empty of stars inside, which makes it a surprisingly good measure of just how dark your sky really is.