June 23
The Rider and the Test
Sun Position
The Sun is in Cancer, declination near +23.3°. Northern days are slowly beginning to shorten following the solstice; twilight still lingers late.
Sky Highlight
No major meteor shower peaks today. The June Bootid meteor shower has an unpredictable but occasionally strong outburst near June 27; observers can begin watching the Boötes region in the final week of June for any activity.
Deep Sky Object
M51 (Whirlpool Galaxy), an interacting spiral galaxy roughly 23 million light-years away. M51 in Canes Venatici is the first galaxy in which spiral structure was directly observed, by Lord Rosse in 1845; it is currently interacting with its companion NGC 5195, and the gravitational disturbance is triggering active star formation in its arms. Well-placed in June from northern latitudes; visible in binoculars as a fuzzy pair, resolved in a small telescope under dark skies.
Featured Star
Alcor (80 UMa) is a main-sequence A-type star (A5V) about 82 light-years away in Ursa Major, paired visually with the much more famous Mizar, the two have been used for centuries as a traditional test of eyesight. Alcor and Mizar are now confirmed to form a genuine six-star system, loosely bound by gravity.
Around This Date
- June 22, 1978James Christy discovered Charon, Pluto's largest moon, identifying a periodic bulge in Pluto's image on photographic plates taken at the U.S. Naval Observatory.
- June 24, 1997A docking accident aboard the Mir space station caused the Progress M-34 cargo vessel to puncture the Spektr module, which was sealed off and never repressurized.
For millennia it was a test of eyesight; now we know six stars are hiding in that corner of the Big Dipper's handle.