September 15
The Southwest Sentinel
Sun Position
The Sun is in Virgo near -1° declination, just south of the celestial equator. The autumnal equinox has recently passed or is imminent; day and night are very nearly equal everywhere on Earth.
Sky Highlight
Mid-September is peak season for observing the Milky Way's northern galactic center region from Northern Hemisphere dark sites, even as it slides toward the western horizon after dark. For Southern Hemisphere observers, the galactic core set in the west weeks ago; attention turns to the rich Sagittarius-Scorpius region now in the west.
Deep Sky Object
M2 (NGC 7089), globular Cluster, Aquarius. About 37,500 light-years away, M2 is one of the larger globular clusters visible to the naked eye as a faint smudge. It is well-placed in the southern sky for Northern Hemisphere observers; high overhead from southern latitudes.
Featured Star
Markab (α Peg) is a blue-white giant of spectral class B9III about 140 light-years away, no longer fusing hydrogen in its core and in the process of evolving into a cooler, expanded state. Its steady blue-white light provides a useful color comparison against the orange-red of Scheat just a few degrees away in the same asterism.
Around This Date
- September 15, 1968Zond 5 became the first spacecraft to travel to the Moon and return to Earth, carrying biological samples including tortoises, wine flies, and plants, an important step toward crewed lunar missions.
- September 17, 1789William Herschel discovered Mimas, a moon of Saturn, during systematic sweeps of the sky with his large reflecting telescope.
Markab holds its corner of the Square without fuss, a reliable anchor for eyes navigating from one asterism to the next.