Zodiac Constellation #1: Gemini

By Peter Forrester | February 6, 2019

Greetings, stargazers!

In case you’re wondering about my choice of constellations, I pointed out in the article about the Zodiac that the constellations each have the Sun in their direction for about one month; although that month is off by a little bit from the traditional astrological dates. I therefore can’t write about it in its normal month because it can’t be seen then. I will be writing about a constellation that is visible in the east in the early evening, around 6 or 7 pm in US Eastern Time.

Gemini is Latin for “twins”, and indeed, the two brightest stars in Gemini are very similar in brightness, if not in detail. Castor and Pollux were twins from Greek mythology, hunters and later sailors on Jason’s Argo. They were said to have been placed together in the sky after one of them was killed. Oddly, they are also often said to have had different fathers, with one of the fathers being the god Zeus.

Pollux is the brightest star in Gemini, with an apparent magnitude of 1.14. It is an orange giant star about 34 light-years from Earth. Castor is just behind it at 1.58, but actually consists of SIX different stars, in three binary pairs. They are 52 light-years away, and appear to be a single blue-white star.

The brightest stars in each constellation are assigned Greek letters, with the brightest one usually being designated as Alpha (α), but Gemini is an exception, with Castor being α Geminorum, or in short form α Gem (you’ll often see this short form next to the stars on sky charts). Pollux, the brightest star, gets the next letter, Beta (β Geminorum or β Gem). After proceeding through all 24 letters of the Greek alphabet, the dimmer stars will be assigned numbers.

The remainder of the constellation principally consists of two lines of stars, one extending out to the right of each of the bright stars (in the northern hemisphere). There are 85 stars in Gemini that are visible without using a telescope. Four of the stars are brighter than 3.00. Eight of the stars are known to have planets, one of them being Pollux.

In drawings of the constellation, Castor and Pollux are the heads of the twins, with the parallel line going down along towards their feet at the other end. Their feet are pointed right at the constellation Orion.

Gemini is easy to find in the sky if you can identify Orion (February is the best month to see Gemini, so I recommend braving the cold). From northern latitudes, it appears just to the east (left) and slightly above Orion. If you make a line from the right side of Orion’s belt, or the right foot, and pass through the left shoulder (from your perspective), it will lead straight to Castor and Pollux.

Gemini is important in astronomical history: the planet Uranus and the former planet, now dwarf planet Pluto, were both discovered in Gemini, in 1781 and 1930, respectively. Much earlier, Aristotle wrote about the earliest recorded occasion of a planet occulting, or passing in front of a star. The planet Jupiter passed in front of a star in Gemini; it is believed that the star was the one called 1 Geminorum (not the 25th brightest – its number comes from a catalog that was done from west to east), and that the event occurred on 5 December, 337 BC.

Next in series: Zodiac Constellation # 2: Cancer


Encyclopaedia Britannica. (2009, February 18). Castor (star). Retrieved from https://www.britannica.com/place/Castor-star.

Wikipedia. (2018, July 27). 1 Geminorum. Retrieved from https://en.wikipedia.org/wiki/1_Geminorum.

Wikipedia. (2019, January 10). Castor (star). Retrieved from https://en.wikipedia.org/wiki/Castor_(star).

Wikipedia. (2019, January 29). Gemini (constellation). Retrieved from https://en.wikipedia.org/wiki/Gemini_(constellation).

Wikipedia. (2019, January 2). Pollux (star). Retrieved from https://en.wikipedia.org/wiki/Pollux_(star).

Author: Peter Forrester

I have been interested in astronomy and stargazing for many years, and now delight to offer some of my learning to others through my weekly blog posts.

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