May 21
The Pulsing Cross
Sun Position
The Sun is in Gemini at roughly +20.3° declination. The Sun has crossed from Taurus to Gemini, a zodiacal transition that historically marked the approach of summer. Northern days are at their longest stretch of the year; southern days are at their shortest.
Sky Highlight
With the Sun now in Gemini, late May offers the best opportunity before summer haze and shorter nights to observe the great globular clusters M3 and M5, both riding high in the south at mid-northern latitudes. From southern latitudes, the rising Scorpion with M4, M6, M7, and M80 commands the evening sky.
Deep Sky Object
M4 (NGC 6121), globular cluster in Scorpius, about 5,600 light-years away. The closest globular cluster to Earth, M4 is unusual in having a bright bar of stars across its core. A white dwarf in M4 was used to derive one of the earliest observational estimates of the age of the universe. Best from southern and equatorial latitudes.
Featured Star
Mimosa (β Crucis) is a blue giant 280 light-years away, spectral class B0.5III, and the second brightest star in the Southern Cross. It is also a Beta Cephei variable: its brightness pulsates with a period of about six hours as its outer layers expand and contract, a cycle driven by the interplay of radiation pressure and gravity in its interior.
Around This Date
- May 21, 1961President Kennedy addressed Congress calling for a national commitment to land on the Moon before the end of the decade, setting in motion the Apollo program.
- May 22, 1969Apollo 10's lunar module Snoopy descended to within 15 kilometers of the lunar surface in a full dress rehearsal for the Moon landing, without actually touching down.
A star that pulsates every six hours keeps a kind of clock.