GAM 2012 Blog

April 17

By Tavi Greiner

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Last year, in expressing the need to be more tolerant of people who seemed to prefer cosmic fallacies to cosmological facts, I promised to write a 12-part series that would introduce astrology fans to astronomy, through monthly articles featuring each of the zodiacal constellations. I begin the series with Aries, this year, during Global Astronomy Month 2012!



Astrologers tell us that the twelve “signs” of the zodiac hold great influence over individuals, specifically by proxy of birth date and according to the procession of the planets. But the signs of the zodiac are much more than personalized horoscopes and tarot card readings; they are a literal illustration of our physical position in the solar system, the Milky Way galaxy, and even the Universe. And this illustration – that position – can be seen and felt by anyone, anywhere, throughout the year.

zodiac-signsThe signs of the zodiac are derived from 12 of 88 “patterned” groups of stars which divide the entire night sky – east, west, north, and south – into recognizable regions called constellations. The twelve zodiacal constellations were specifically defined in the creation of a 360˚ celestial coordinate system which uses the ecliptic – that visible path along which the Sun, Moon, and planets traverse the sky - as its fundamental plane. In other words, the twelve zodiacal constellations are aligned along a specific apparent path encircling our planet.

As the Sun, Moon, and planets pass along the ecliptic plane, thus through each of the zodiacal constellations – there are actually thirteen - they reveal their (and our!) relative positions in the solar system, not their influence in our lives. In addition to watching the individual orbits of planets within our solar system, we can also use the zodiacal constellations – in fact, all of the constellations – to experience the immenseness, diversity, and even the evolution, of our entire Universe.

venus-and-crescent-moonThe important point is not whether or not you find some spiritual connection to the stars, but that you do embrace the night sky, as much for its physical and visual qualities as for its believed influence. For it is a greater understanding of the physical reality, rather than any adherence to a mystical evocation, that most enlightens our path toward tomorrow’s horizons.

So, go ahead and check your horoscope to see what your “sign” suggests about your future, and then grab your binoculars to see what your “constellation” really reveals.

Edition 1 of 12
March 21 – April 19


Our first stop along the zodiac is “courageous” Aries. It’s not a particularly spectacular region, but it does hold the distinction of being the first constellation in the zodiac, in accordance to the Sun’s vernal equinox position when the zodiac was first created, many thousands of years ago. Today, the Sun’s vernal equinox position is a little further along the ecliptic (so Aries people are actually Pisces,) but Aries has maintained its 1st place zodiac position.


Constellation Aries is represented by 4 stars – Mesarthim, Sheratan, Hamal, and 41 Ari - stretched along a crook’d line which, in ancient Babylonian, Arabic, and Greek cultures, represented a resting sheep or ram. In the northern hemisphere, Aries is best seen in winter months, but you can still catch it in very early Spring, for about an hour after sunset.

Aries is positioned between neighboring constellations Perseus, Taurus, Cetus, Pisces, and tiny Triangulum. Mesarthim and Sheratan - the crook’d pair at the ram’s head - point towards Pisces; Aries’ brightest star, Hamal, sits at the ram’s shoulder; and 41 Ari - the ram’s tail - points towards Perseus and Taurus.

hamalHamal, aka Alpha Arietis or 13 Ari, is a semi-bright K-type orange-giant star located some 65-light-years-away. It is about 15-times the diameter of our Sun, twice as massive, 90-times as luminous, and a bit cooler. Hamal’s spectrum is used to classify other stars, and it is one of only a few stars in which limb darkening has been seen. Hamal is believed to host at least one planet – a gas-giant nearly twice the size of Jupiter with an orbital period of just over one year.

Aries beta star, Sheratan or 6 Ari, is actually a pair of closely-orbitingstars located about 60-light-years-away. The primary star is an A-type main sequence star, and the secondary star, as yet unresolved, appears to be an F- or G-class star.

mesarthimAries’ third brightest star, Mesarthim or Gamma Ari, is comprised of two faint stars which, together, give the appearance of one slightly-less-faint star. Unlike Sheratan’s two-star system, this pair can be seen (through a small telescope) as the blue and yellow individuals that they are. Gamma 1 is a class-B dwarf; Gamma 2 is a highly-magnetized class-A dwarf or class-B subgiant; and each are roughly 2.5 times the size of our Sun. And though they appear close to each other, the two are separated by more than 46-billion miles and require 5000 years to complete one orbit around each other.

The final star of Aries, that unnamed tail simply designated as 41 Ari, is actually slightly brighter than Mesarthim. This B-class dwarf star once belonged to an ancient constellation known as Musca Borealis, thus its designation of Aries’ 4th star, rather than the 3rd. 41 Ari is a fast-spinning star about 2.5 times the radius of our Sun and is located about 160-light-years-away.

pleiadesMany constellations are host to some of the brighter nebulae and star clusters in our galaxy, but Aries is a smaller constellation, offering only its outlining stars and a few fainter galaxies visible only through a telescope. You can, however, enjoy some beautiful objects in neighboring constellations, including the Pleiades and Hyades star clusters in Taurus, the Double Cluster in Perseus, and the Great Andromeda Galaxy in (where else, but) Andromeda.

There you have it, the starlit realm of Aries – as it really exists, in the Universe that surrounds us! Next month, we’ll feature “dependable” Taurus – home to the famous Pleiades and Hyades clusters - for those born between April 21 and May 21.


Tavi Greiner is an amateur astronomer who devotes much of her time to astronomy outreach, most recently as an astronomy writer for a local weekly newspaper and as VP and Communications Director for Astronomy.FM. She is also a co-producer of A Sky Full of Stars astronomy productions and has held previous producer positions with SLOOH and Astrocast.TV. You can follow Tavi's latest astronomy exploits on Twitter, Zenfolio, and Flickr.