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Gemini, a constellation for February

Contributed by Paul Rodmell, Southland Astronomical Society

GEMINI (pronounced JEM'n-eye) the Twins

Chart showing the constellation.

This constellation dates from ancient times, representing a pair of twins, the sons of Leda, holding hands. We know these twins as Castor and Pollux, members of the Argonaut's crew. They are half brothers since their fathers were King Tyndareus of Sparta and Zeus, respectively. The twins were the patron saints of mariners, appearing in the ship's rigging as St. Elmo's Fire. In ancient times, Ptolemy assigned eighteen stars to Gemini.

The Geminid meteor shower appears to radiate from a point in near Castor. This shower is one of the richest and most brilliant during the year, reaching a maximum around December 14th.

To find Gemini look north in the late evening sky, and find the bright belt stars of Orion marking the base of the "Pot". Look to the right to find the bright star Procyon in Canis Minor, and then drop down to find the bright pair of Castor and Pollux.

Chart showing Gemini - as seen about 11.00 pm (NZDT) mid February.

The chart shows the horizon from the south of New Zealand. For places further north, the horizon would be lower on the chart, that is the constellations higher. Some stars hidden for southern observer, would be just above the horizon for more northerly observers.

Gemini. 9.7 kb

Constellation Eridanus Constellation Orion Constellation Monoceros Constellation Hydra Constellation Sextans Constellation Leo Constellation Cancer Constellation Canis Minor Constellation Leo Minor Constellation Lynx Constellation Auriga Constellation Tuarus

Details of some of the objects shown in the chart.

α Geminorium, Castor is an astounding multiple star of six components. To the unaided eye it appears as a blue-white magnitude 1.6 star 52 light years away, but a small telescope will split this into two close components of magnitudes 2.0 and 2.9, which orbit each other in 467 years. Both stars are spectroscopic binaries, and small telescopes show a 9th magnitude red dwarf companion, which is itself an eclipsing binary star.

β Gem, Pollux at magnitude 1.1, is the brightest star in the constellation despite being "beta". This is mainly because Johann Bayer, who labelled the stars with Greek letters in 1603, did not carefully distinguish which was the brighter star of the "Twins". Pollux is 33.7 light years away.

γ Gem, is a magnitude 1.9 blue-white star 105 light years away.

δ Gem is a creamy-white star of magnitude 3.5 with a reddish magnitude 8.1 companion. The contrast between the brighter and faint companion make the pair difficult in small apertures. The brighter star is a spectroscopic binary with a 6.13 year period.

ε Gem is a magnitude 3.0 yellow supergiant star 903 light years away. Powerful binoculars or a small telescope reveal a 9th magnitude companion.

ζ Gem is both a variable star and a binocular double star. The yellow super-giant star is Cepheid variable 1170 light years away, fluctuating between magnitude 3.7 and 4.1 every 10.2 days. Binoculars or small telescopes reveal a wide 8th magnitude unrelated companion star.

η Gem is 349 light years away and another double variable star. This red giant star fluctuates in a semi-regular manner between magnitudes 31 to 3.9 every 230 days. It has also a close 8th magnitude companion, requiring a large aperture telescope to distinguish it from the glare of the primary star. The primary star is a spectroscopic and occultation binary with a period of 8.17 years.

κ Gem is a magnitude 3.6 yellow giant star 143 light years away, with a magnitude 9.5 companion star difficult in small telescopes because of the brightness contrast.

ν Gem is a blue-white star 503 light years away with a wide 9th magnitude companion.

38 Gem is a beautiful double star for small telescopes, shining like gems in a field sown with scattered stars. Its white and yellow components are of magnitude 4.7 and 7.6 lying 1500 light years away.

M 35 (NGC 2168) is a large bright star cluster visible to the unaided eye from a dark sky site. Binoculars or small telescopes at low magnification show the 200 or so stars that make up this cluster to be in curving chains. It lies about 2600 light years away.

NGC 2392 is a large moderately bright 8th magnitude planetary nebula, known as the Eskimo or Clown Face Nebula, because of its appearance in large telescopes or photographs. Small telescopes show it as a fairly large rounded blue-green disc. Its central star is of 10th magnitude.


Gemini is north of the equator so fairly low for New Zealand observers. The centre of the constellation is due north at about 10.00 pm (NZDT) in mid of February. The brightest star, Pollux, β Gem, is due north about an hour later. For New Zealand viewers, with the Sun setting earlier during the following three months, Pollux will remain visible as soon as the sky is dark after sunset. It will be to the north during March and early April, but will gradually move round to the north-west and get lower during the rest of April and May.

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