October 25

October 25

The Former End

Sun Position

The Sun is in Scorpius near -15.5° declination. The solar disk is now setting distinctly south of west across Northern Hemisphere mid-latitudes, and rising noticeably north of east across the Southern Hemisphere.

Sky Highlight

Late October is the onset of the best season for observing the Fornax Galaxy Cluster, which becomes accessible in the evening sky over the southern horizon from Northern Hemisphere mid-latitudes and high in the sky for Southern Hemisphere observers. The cluster contains dozens of galaxies within a two-degree field.

Deep Sky Object

NGC 1316 (Fornax A), a lenticular galaxy / radio galaxy about 70 million light-years away. NGC 1316 is the brightest radio source in Fornax and shows the tangled shell structures of multiple past galaxy mergers, its distorted outer envelope records several billion years of collisions that built it into one of the most massive galaxies in the southern sky. Best for Southern Hemisphere observers; visible from northern mid-latitudes but low.

Featured Star

Acamar (θ Eri) is a white giant and white main-sequence pair (A4III + A1V) about 161 light-years away. Its name is a variant of Achernar, and it was itself called the river's end in older star catalogs, until explorers in the Southern Hemisphere mapped the sky far enough south to reveal that Eridanus continued on to the star now called Achernar, demoting Acamar to a way-station. Acamar, the former end of the river, displaced when the deeper south was mapped.

Around This Date

  • October 25, 1671Giovanni Cassini discovered Iapetus, Saturn's moon with a striking two-toned surface (one hemisphere nearly as bright as snow, the other nearly as dark as charcoal) which he could not explain and which remained puzzling for three centuries.
  • October 28, 1971Britain's Prospero satellite reached orbit on a domestic Black Arrow rocket, making the UK the sixth country to achieve independent orbital launch capability.

Acamar was renamed by history, not by astronomy, the sky's edges moved when human observation finally reached them.