May 27
The First Double
Sun Position
The Sun is in Gemini at roughly +21.4° declination. The Northern Hemisphere is in its warmest and longest days; summer begins in less than a month. Southern days are at their shortest, with the winter solstice approaching.
Sky Highlight
The Big Dipper is nearly overhead for northern mid-latitude observers in late May, making this an ideal time to observe the Mizar-Alcor pair in the handle. In the same evening, M51, M63, M101, and M106 are all accessible at high elevation, a compact group of bright galaxies, all belonging to a loosely defined local group of galaxies called the Canes Venatici Cloud.
Deep Sky Object
M106 (NGC 4258), spiral galaxy in Canes Venatici, about 23.5 million light-years away. It has an anomalous nucleus with two extra spiral arms that appear in radio wavelengths as jets of material from its central black hole. In optical telescopes it appears as a large, bright spiral with a noticeable elongated core. Best from northern and equatorial latitudes.
Featured Star
Mizar (ζ Ursae Majoris) is a white main-sequence pair 83 light-years away, spectral class A1V + A1V. It holds a specific place in the history of astronomy: in 1617, Giovanni Battista Riccioli observed through a telescope and recorded that Mizar was not one star but two, making it the first double star ever resolved telescopically. The pair also forms part of a larger sextuple system with nearby Alcor.
Around This Date
- May 27, 1931Auguste Piccard and Paul Kipfer made the first manned stratospheric balloon ascent, reaching an altitude of about 15.8 kilometers, a direct precursor to high-altitude atmospheric and cosmic-ray research.
- May 29, 1919Arthur Eddington and Frank Dyson photographed stars near the Sun during a total solar eclipse and confirmed that starlight was bent by the Sun's gravity as predicted by Einstein's general theory of relativity.
The first telescopic double star: someone looked carefully and found two where there had been one.