April 13
The Retired Pole Star
Sun Position
The Sun is in Aries near +16° declination. Northern evenings are long and the constellation Draco is high in the northeast after dark; southern hemisphere observers see Draco low or below the horizon.
Sky Highlight
April evenings are prime for observing the northern circumpolar constellations, Ursa Major, Ursa Minor, Draco, and Cassiopeia all remain above the horizon all night from latitudes above 45°N. Draco in particular winds between the two bears and is well placed for tracing its full length on clear April nights.
Deep Sky Object
NGC 4631, the Whale Galaxy, edge-on spiral in Canes Venatici, about 25 million light-years. The Whale Galaxy is a large, distorted edge-on spiral with a smaller companion, NGC 4627; the gravitational interaction has triggered star formation visible as bright knots along the disk. Best from northern mid-latitudes; not well placed from southern locations.
Featured Star
Thuban (α Dra) is a white A0III giant 303 light-years away, a star whose principal claim on our attention is historical rather than intrinsic. Earth's axial precession carried the north celestial pole to within 0.2° of Thuban around 2787 BCE, when Egyptian pyramid builders may have used it as a polar reference; now it sits 26° from the pole and Polaris holds the role.
Around This Date
- April 13, 1742Edmond Halley, the British astronomer who predicted the return of the comet bearing his name, died in Greenwich; he had been Astronomer Royal since 1720.
- April 14, 2010The European Southern Observatory released images from the VISTA survey telescope in Chile, demonstrating its wide-field infrared capability for mapping star-forming regions.
Thuban was the pole star while the pyramids were being built; it has since slipped 26 degrees from that position, and the Earth is not finished moving.