October 20
The 1664 Double
Sun Position
The Sun is transitioning from Libra toward Scorpius near -13° declination. The Northern Hemisphere has lost nearly two hours of daylight since the September equinox; the Southern Hemisphere has gained the same.
Sky Highlight
The Orionid meteor shower, associated with debris from Halley's Comet, is active from approximately October 2 to November 7, with peak rates typically occurring around October 21-22. On October 20, activity is near its maximum, expect 10 to 20 meteors per hour under dark skies, best viewed after midnight when the radiant in Orion is well above the horizon. Visible from both hemispheres.
Deep Sky Object
M15, a globular cluster about 33,600 light-years away. M15 in Pegasus is one of the most densely packed globular clusters known and the first to be shown to contain a planetary nebula (Pease 1) embedded within its stellar swarm, discovered in 1928 and still one of only a handful of such objects found inside a globular. Well-placed for Northern Hemisphere observers in October; accessible from Southern Hemisphere but lower on the northern horizon.
Featured Star
Mesarthim (γ Ari) is a blue-white main-sequence pair (B9V + A1V) about 164 light-years away. Its historical significance sits in a peculiar footnote: Robert Hooke observed and recorded it as a double star around 1664, making it one of the first stellar duplicities discovered telescopically, not because anyone was looking for doubles, but as a by-product of tracking a comet. Mesarthim, one of the first double stars ever seen through a telescope, in 1664.
Around This Date
- October 21, 1988The International Astronomical Union formalized the name 'Mesarthim' for γ Arietis, part of a broader effort to standardize star names that had accumulated centuries of variant spellings.
- October 22, 1797André-Jacques Garnerin made the first parachute descent from a balloon at high altitude over Paris, a precursor to technologies later essential in spacecraft recovery.
The Orionids peak tomorrow, but tonight the radiant is already high enough that patient watching after midnight is worth the effort.