September 26

September 26

The Sea Monster's Tail

Sun Position

The Sun is in Libra near -3° declination. Northern Hemisphere evenings are now perceptibly longer than mid-September; Southern Hemisphere spring afternoons are extending noticeably.

Sky Highlight

Cetus, the sea monster, begins rising fully above the eastern horizon in the evening hours this week, and Mira (o Ceti) (one of the sky's most famous long-period variable stars) can be anywhere from naked-eye brilliant to near-invisible depending on where it is in its 332-day cycle. Worth noting at any time of year that Mira's current brightness is worth checking before observing.

Deep Sky Object

M77 (NGC 1068), Seyfert Galaxy, Cetus. About 47 million light-years away, M77 is one of the closest and most studied active galactic nuclei in the sky; its central black hole powers a compact, high-energy core that outshines many larger galaxies' total stellar light. Well-placed in the southeast by late evening for Northern Hemisphere observers; accessible from mid-southern latitudes.

Featured Star

Diphda (β Cet) is an orange giant of spectral type K0III about 96 light-years away, the genuinely brightest star in Cetus despite its Beta designation, a labeling artifact from early catalog work that assigned names before all stars in a constellation were measured carefully. It is a mild variable with a low-amplitude long-period variation.

Around This Date

  • September 26, 1983Soviet early-warning officer Stanislav Petrov correctly identified a satellite sensor error as the likely cause of a false alarm showing five incoming intercontinental ballistic missiles, and did not forward the alert, a decision widely credited with helping to avert a catastrophic misunderstanding.
  • September 28, 2015NASA announced that recurring slope lineae on Mars were strong evidence of contemporary liquid water activity on the Martian surface, based on CRISM spectroscopy data from the Mars Reconnaissance Orbiter.

Diphda is a star that got a secondhand name for holding first place, a reminder that the sky's catalog was built by humans working in sequence, not simultaneously.