March 15
The Hinge
Sun Position
The Sun is in Pisces, declination near +14.5°; the Northern Hemisphere is within five days of the vernal equinox, and the rate of daily change in sunrise and sunset times is near its annual peak.
Sky Highlight
The Big Dipper is now nearly overhead from mid-northern latitudes by 10 PM local time, its bowl tipped in the characteristic spring orientation with the handle arcing eastward. This position of the Dipper has been used for centuries as a seasonal clock: 'the handle points east, it's spring.'
Deep Sky Object
M101 (Pinwheel Galaxy), a face-on spiral galaxy in Ursa Major, about 21 million light-years away, large and low surface brightness; it requires dark skies to see the spiral arms, but its disk is visible in small telescopes as a dim glow near the end of the Dipper's handle. Northern Hemisphere object, poorly placed from southern latitudes.
Featured Star
Megrez (δ UMa) is a main-sequence A-type star 58 light-years away with spectral class A3V, connecting the bowl to the handle of the Big Dipper at the hinge star. It is the faintest of the seven Dipper stars (about three times less luminous than its neighbors) which has historically led to speculation, now disfavored, that it may have faded since antiquity.
Around This Date
- March 11, 1811Urbain Le Verrier was born; he would later predict the position of Neptune mathematically in 1846 before it was directly observed, and identify the anomalous precession of Mercury's orbit that would not be explained until Einstein's general relativity in 1915.
- March 15, 1713Nicolas-Louis de Lacaille was born; he would go on to catalog over 10,000 southern stars from the Cape of Good Hope between 1750 and 1754 and name 14 of the 88 modern constellations, most after scientific instruments.
The hinge between bowl and handle: Megrez is the quiet pivot of one of the most recognized shapes in the Northern sky.