Thursday, November 20, 2008

The Reclusive Ram

Aries the Ram is a bit of a recluse, hiding in plain sight in the late autumn sky. Although not terribly impressive to behold, Aries is historically an important constellation. In the astrological zodiacs of numerous ancient civilizations, it was considered the first zodiacal constellation, or first sign. This designation may have been made when it was noted by the ancients that the Sun was positioned in Aries at the spring equinox.

The spring equinox is the moment when the center of the Sun is directly above the Earth’s equator. This occurs only twice a year; the other occurrence is known as the fall equinox. Another name sometimes given for the spring equinox is first point of Aries, which, of course, derives from old astrological traditions.

Aries in Bayer's 1603 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

The “first sign” tradition continues today; Aries is usually listed first in horoscope columns found in newspapers and magazines. You’ll also note that the conventional date range given by astrologers for people “born in the sign of Aries” begins on March 21; spring equinox typically occurs on or near March 20.

By the way, the Sun is no longer in Aries at the spring equinox. Due to precession— sometimes called precession of the equinoxes— which is the wobbling-top motion of Earth’s axis, the equinoxes occur a few minutes earlier each year. As a result, at each spring equinox, the Sun is not in the same position it was the previous year against the background stars, as seen from Earth. If we track the Sun’s position over the years at a fixed point in time (such as the spring equinox), from our perspective on Earth the Sun would appear to move through all 12 zodiacal constellations in reverse. As a result of precession, the Sun is now in Pisces, the twelfth zodiacal constellation, at the spring equinox.

Aries in Middleton's 1842 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

In mythology, Aries represents the ram whose golden fleece was famously sought by Jason and the Argonauts. Let’s conduct our own search for the storied sheep.

1) Wait about two hours after sunset to begin observing, so that Aries is high enough in the sky.

2) Face east. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your back to the west, and you’ll be facing approximately east.

Star maps created with Your Sky

3) Find the Great Square of Pegasus, which is nearly overhead, and the Pleiades star cluster, which is low in the east. These are both easy to spot using the star map above. Midway between them is Aries. Look for a small, curved line of three stars. This is the most notable asterism (recognizable star pattern) in Aries. It always remind me of a comma, so I call this asterism the Comma. Unfortunately, there’s nothing in the scattering of stars in Aries that particularly resembles a ram, unless you want to think of the Comma as a curving ram’s horn.

4) The brightest star in Aries is Hamal (hah-MAHL), which comes from the Arabic for lamb. Other ancient names for this star had meanings like ram’s eye, horn star, and leading one (the ram that led the celestial flock). Hamal is an orange giant star, 66 light years away. One light year is the distance light travels in one Earth year, nearly six trillion miles.

5) The second brightest star in Aries is Sheratan (SHER-uh-tunn), from the Arabic for the pair, a reference to Sheratan and the final star in the Comma, which we’ll look at next. Sheratan is a white star. When you compare it to Hamal, can you see that orange giant Hamal has a more golden hue?

6) Mesarthim (mess-ahr-TEEM) is from the Hebrew for servants. Mesarthim, a white star, may only be Aries’ fourth brightest but it’s known as the first star of Aries. This is because in antiquity, when the spring equinox was in Aries, Mesarthim was the closest Ram star to the equinox. Physically, the equinox can be described as the intersection of the Sun’s path with the celestial equator, the imaginary line that represents the Earth’s equator extended out into space.

7) Although the third brightest star in Aries has no traditional name and is instead saddled with the uninspired moniker Flamsteed 41, it does manage to be part of a funky little asterism. The blue-white dwarf star is the brightest member of a four-star asterism known as the Northern Fly, riding on the ram’s back. To find Flamsteed 41 and the Northern Fly, look northeast of Hamal and east of the constellation Triangulum.

In its glory days, this asterism was part of a now-defunct constellation by the same name, which was introduced in the 17th century. Now it’s merely a curiosity for naked-eye stargazers like us and, I imagine, a constant irritant for our celestial ram.

The Ram and the Fly in Bode's 1801 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology