April 15
The Bear's Guardian
Sun Position
The Sun is in Aries near +18° declination. Northern hemisphere spring is in full force; in the southern hemisphere, the Sun's altitude at noon is declining noticeably toward winter.
Sky Highlight
Arcturus becomes prominent in April evenings, a useful annual marker: when Arcturus is well up in the east after dark, spring galaxy season is fully open. Follow the arc of the Big Dipper's handle to Arcturus (Arc to Arcturus), then continue the curve to Spica, a two-step navigation used by amateur astronomers across the northern hemisphere.
Deep Sky Object
M63, the Sunflower Galaxy (NGC 5055), spiral galaxy in Canes Venatici, about 29 million light-years. M63 is a flocculent spiral, its arms are patchy and diffuse rather than sharply defined, with a bright nucleus visible in small telescopes and outer arms showing well in long-exposure images. Best from northern mid-latitudes.
Featured Star
Arcturus (α Boo) is a K1.5 red giant 36.7 light-years away, the brightest star in the northern celestial hemisphere and the fourth brightest in the entire sky. It is a thick-disk star moving at high velocity relative to the Sun, evidence that it formed in an older stellar population and is just passing through the Sun's neighborhood on a different galactic orbit.
Around This Date
- April 15, 1452Leonardo da Vinci was born; his notebooks contain observational drawings of the Moon's surface and notes on the nature of moonshine, including the correct explanation of Earthshine.
- April 17, 1970Apollo 13's crew safely splashed down in the Pacific Ocean after using the lunar module Aquarius as a lifeboat following the oxygen tank explosion six days earlier.
Arcturus is crossing the sky on a different trajectory than the Sun and its neighbors; in a few million years it will be gone from our sky, a brief bright guest.