May 20
The Grain in the Hand
Sun Position
The Sun moves into Gemini on or around May 20 (the exact date varies by a day year to year). Its declination reaches about +20.1°. This is the last full day in Taurus for most years. Northern days are very long; southern days are near their minimum.
Sky Highlight
The Sun's entry into Gemini marks the final stretch toward the June solstice. In the Northern Hemisphere, this is the period of maximum twilight: civil, nautical, and astronomical twilight overlap so heavily at high latitudes that true astronomical darkness may last only a few hours, or not occur at all above 60°N. Observing windows are short but intense.
Deep Sky Object
M104 (NGC 4594), the Sombrero Galaxy in Virgo, about 28 million light-years away. Its edge-on orientation and prominent dark dust lane make it one of the most photographed galaxies in the sky. It is detectable in binoculars as an elongated smudge from a dark site. Equatorial and southern observers have the best altitude in May.
Featured Star
Spica (α Virginis) is a blue giant and blue main-sequence binary 250 light-years away, spectral class B1III-IV + B2V. The two components orbit each other every four days, close enough that tidal forces distort them into ellipsoids, a fact that Hipparchus exploited around 127 BCE when he compared its position to earlier star catalogs and became the first person to measure the precession of Earth's axis.
Around This Date
- May 20, 1990Hubble Space Telescope images revealed spherical aberration in the mirror, a manufacturing error that would be corrected by astronauts in December 1993.
- May 20, 1978Pioneer Venus Orbiter launched from Cape Canaveral, becoming the first spacecraft to systematically study the Venusian atmosphere from orbit.
Spica gave Hipparchus the wobble of the Earth; the grain holds the measure of the world.