Gemini is one of the constellations of the zodiac. It was one of the 48 constellations described by the 2nd century AD astronomer Ptolemy, and it remains one of the 88 modern constellations today. Its name is Latin for “twins,” and it is associated with the twins Castor and Pollux in Greek mythology. Gemini lies between Taurus to the west and Cancer to the east, with Auriga and Lynx to the north and Monoceros and Canis Minor to the south. The Sun resides in the astrological sign of Gemini from June 20 to July 20 each year (though the zodiac dates that it’s May 21 to June 21). By mid-August, Gemini will appear along the eastern horizon in the morning sky before sunrise. The best time to observe Gemini at night is overhead during January and February. By April and May, the constellation will be visible soon after sunset in the west. The easiest way to locate the constellation is to find its two brightest stars Castor and Pollux eastward from the familiar “V” shaped asterism of Taurus and the three stars of Orion’s belt.
The constellation contains 85 stars. The brightest star in Gemini is Pollux, and the second-brightest is Castor. Castor’s Bayer designation as “Alpha” arose because Johann Bayer did not carefully distinguish which of the two was the brighter when he assigned his eponymous designations in 1603. α Gem (Castor) is a sextuple star system 52 light-years from Earth, which appears as a magnitude-1.6 blue-white star. Two spectroscopic binaries are visible at magnitudes 1.9 and 3.0 with a period of 470 years. A wide-set red dwarf star is also a part of the system; this star is an Algol-type eclipsing binary star with a period of 19.5 hours; its minimum magnitude is 9.8, and its maximum magnitude is 9.3. β Gem (Pollux) is an orange-hued giant star of magnitude 1.14, 34 light-years from Earth. Pollux has an extrasolar planet revolving around it, as do two other stars in Gemini, HD 50554, and HD 59686. γ Gem (Alhena) is a blue-white hued star of magnitude 1.9, 105 light-years from earth. δ Gem (Wasat) is a long-period binary star 59 light-years from Earth. The primary is a white star of magnitude 3.5, and the secondary is an orange dwarf star of magnitude 8.2. The period is over 1000 years. ε Gem (Mebsuta), a double star, includes a primary yellow supergiant of magnitude 3.1, nine hundred light-years from Earth. The optical companion, of magnitude 9.6.
Since the sky area of Gemini is directed away from the Milky Way, there are comparatively few deep-sky objects of note. M35 (NGC 2168) is a large, elongated open cluster of magnitude 5, discovered in the year 1745 by Swiss astronomer Philippe Loys de Chéseaux. It has an area of approximately 0.2 square degrees, the same size as the full moon. The 200 stars of M35 are arranged in chains that curve throughout the cluster; it is 2800 light-years from Earth. Another open cluster in Gemini is NGC 2158, more than 12,000 light-years from Earth. The Eskimo Nebula or Clown Face Nebula (NGC 2392) is a planetary nebula with an overall magnitude of 9.2, located 4,000 light-years from Earth. It is named for its resemblance to the head of a person wearing a parka. The Medusa Nebula is another planetary nebula, some 1,500 light-years distant. Geminga is a neutron star approximately 550 light-years from Earth. Other objects include NGC 2129, NGC 2158, NGC 2266, NGC 2331, NGC 2355, and NGC 2395.
The Geminids are a bright meteor shower that peaks on December 13–14. It has a maximum rate of approximately 100 meteors per hour, making it one of the richest meteor showers. The Epsilon Geminids peak between October 18 and October 29 and have only been recently confirmed. They overlap with the Orionids, which make the Epsilon Geminids difficult to detect visually. Epsilon Geminid meteors have a higher velocity than Orionids. Credit: Wikipedia.