March 13
The Herald Rises
Sun Position
The Sun is in Pisces, declination near +13.5°; Northern Hemisphere observers are a week from the equinox, and the season's shift is tangible in the evening twilight.
Sky Highlight
On the evening of March 13 (or within a day), look west after dusk. Orion and Canis Major are still clearing the horizon, but they are leaving. Mirzam, the Herald, serves as an annual marker: when it sets not long after Sirius, winter's brightest stars are nearly done for the year.
Deep Sky Object
M41, an open cluster in Canis Major, about 2,300 light-years away, visible to the naked eye just 4° south of Sirius; one of the few Messier objects Aristotle may have recorded, it contains about 100 stars resolvable in binoculars. Best from both hemispheres while Canis Major is still clear of twilight in early evenings.
Featured Star
Mirzam (β CMa) is a blue-white giant 500 light-years away with spectral class B1II-III, and it is also a slowly pulsating B-type star, its brightness varies subtly on a timescale of days as pressure waves cross its interior. In Arabic astronomical tradition it was the herald or announcer, always rising just before Sirius and serving as the sky's advance notice that the brightest star was coming.
Around This Date
- March 13, 1781William Herschel discovered Uranus from his garden in Bath, England, describing it first as a comet before orbital calculations confirmed it was a new planet.
- March 13, 1930Clyde Tombaugh at the Lowell Observatory confirmed the discovery of Pluto after comparing photographic plates taken on January 23 and 29, 1930.
The Herald sets a little earlier each week in March, and when it's finally gone Sirius will follow, winter departing in the order it arrived.