March 24
The Six-Hour Pulse
Sun Position
The Sun is in Aries at roughly +2.5° declination; daylight and darkness are diverging at their fastest rate in the Northern Hemisphere as the Sun moves northward through its most rapid declination change.
Sky Highlight
The Southern Cross is well-placed in southern autumn evening skies, and Mimosa (the cross's second-brightest star) is a fine target for anyone with even a small telescope, as it is also a close visual double. The cross itself is one of the most distinctive asterisms in the sky, featured on the flags of five nations.
Deep Sky Object
NGC 4755 (Jewel Box Cluster) returns as the deep-sky companion here, an open cluster 6,400 light-years away nestled against Mimosa in Crux, containing hot blue-white stars and one contrasting red supergiant. A small telescope resolves it into individual points in a tight, colorful group. Best from latitudes below +25°.
Featured Star
Mimosa (β Cru) is a blue giant 280 light-years away with spectral class B0.5III, and it is a Beta Cephei variable, pulsing in brightness by a small but detectable amount every 6 hours as pressure waves pass through its interior. It is also a double star, with a companion roughly 44 arcseconds away that is probably physically associated.
Around This Date
- March 24, 1893Walter Baade was born in Germany; his observations at Mount Wilson and Palomar led to the resolution of the distance-scale problem in cosmology when he distinguished Population I and II stars in Andromeda, roughly doubling the known size of the universe in 1952.
- March 24, 1965Ranger 9 crashed into the Moon as planned, transmitting thousands of close-up images before impact, the last of the Ranger series and the first to be broadcast live on US television.
Mimosa pulses every six hours, too faint a change for the naked eye but real, a star with its own rhythm, carrying the Cross's second light.