The Heavenly Twins: Find the Sons of Zeus and Leda

The Heavenly Twins: Find the Sons of Zeus and Leda
The scene this time of year at 10:30 p.m. from mid-northern latitudes.

If you head outside around 10:30 p.m. local time this week and look straight up over your head, you'll be looking directly at Gemini, the Heavenly Twins.

The heads of the Twins are the bright stars Pollux (yellowish) and Castor (white; a bit dimmer than Pollux). 

The Twins were the sons of Zeus and Leda and brothers of that Helen whose face launched a thousand ships and caused the Trojan War.  Ancient mariners regarded Pollux and Castor as the patrons of seafarers and in Elizabethan times they were also considered the protectors of all at sea.  The expression "By Jimminy" is a popular corruption of the swearing by the ancients by these patrons, as in "by Gemini."  The brothers have figured in scores of ancient folk tales.  They were adventurers, warriors, and famous navigators.

The stars that compose their arms and bodies are fainter than those in their heads and feet.  A second magnitude star known as Alhena marks one of Pollux' feet.  In places where light pollution hides many of the fainter stars only Pollux, Castor and Alhena may be visible, forming a long wedge with its point aimed straight at Orion, the Mighty Hunter. 

Castor is actually a system of six stars, forming one of the most remarkable examples of a multiple star in the heavens. 

Pollux too, may have faint companions, though we now know that none of them are physically related to this bright star.  Zeta Geminorum is a pulsating Cepheid star, its brightness changing nearly a full magnitude down from 4.4 and back in about 10 days.  Propus is a complex system, a visual binary of magnitudes 3.3 and 6.5, with a period of about 500 years; the brighter member is a semi-regular variable, with an average period of 233 days.  Additionally, an unseen companion star also periodically eclipses this star at intervals of 2,983 days.

Just above and to the right of Propus lies number 35 in Charles Messier's catalogue. Located just off the trailing foot of Castor, M35 can just be seen with the unaided eye on dark transparent nights.  In low-power binoculars it may look like a dim, fairly large unresolved interstellar cloud, but look again.  Even through light-polluted suburban skies, 7x glasses reveal at least a half dozen of the cluster's brightest stars against the whitish glow of about 200 fainter ones. 

M35 has been described as a "splendid specimen" whose stars appear in curving rows, reminding one of the bursting of a skyrocket. 

Walter Scott Houston (1912-1993) wrote the Deep-Sky Wonders column in Sky & Telescope magazine for nearly half a century.  He called M35:  "One of the greatest objects in the heavens; a superb object that appears as big as the Moon and fills the eyepiece with a glitter of bright stars from center to edge." 

Also located less than half a degree southwest from M35 is an unusual object that has brought a brief surge of excitement to countless numbers of amateurs over the years.  In a 6-inch telescope, it's a faint, circular cloud of light, which initially appears to the uninitiated as a possible new comet.  The object, in fact, is the faint open star cluster NGC 2158. 

The compilers of many of the more popular star atlases had to draw a cutoff line as to what deep sky objects to include, and not to include.  Unfortunately, NGC 2158 fell just below the cutoff in most cases, which is why it is usually not mentioned and why many amateur astronomers have grown up knowing nothing of it. 

But if you get a chance to train a telescope on M35 and come across this small, faint patch of nebulosity a short distance away at least you won't be making the "comet mistake" that so many others have made.  Houston himself fell into this trap, later calling NGC 2158 his "lasting monument to my early, somewhat careless, years of observing." 

I am not embarrassed to relate this personal episode: More than 20 years ago, in September 1985, while camping in the Adirondacks of northern New York, I was scanning the Orion-Gemini region with my 10-inch telescope, looking for Halley's Comet, which was then approaching the Sun and the Earth.   It was then that I too, stumbled across NGC 2158.  For not a few minutes I thought I was looking at "Comet Rao" but after composing myself and checking Volume Two of Burnham's Celestial Handbook, I (reluctantly) came back down to Earth. 

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.

Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees. The distance from the horizon to the overhead point (called the zenith) is equal to 90 degrees.

Declination is the angular distance measured in degrees, of a celestial body north or south of the celestial equator. If, for an example, a certain star is said to have a declination of +20 degrees, it is located 20 degrees north of the celestial equator. Declination is to a celestial globe as latitude is to a terrestrial globe.

Arc seconds are sometimes used to define the measurement of a sky object's angular diameter. One degree is equal to 60 arc minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If the disk of Mars is 20 arc seconds across, we can also say that it is 1/90 the apparent width of the Moon (since 1800 divided by 20 equals 90).

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Joe Rao
Skywatching Columnist

Joe Rao is's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.