Aries is one of the constellations of the zodiac, situated north of the celestial equator. It is one of the 88 official constellations as recognized by the International Astronomical Union, albeit as a specific region of sky as opposed to a particular arrangement of stars. It was also listed as one of the 48 recognized constellations of the second century, by astronomer Ptolemy. It ranks 39th in overall size, and lies between the zodiac constellations Pisces and Taurus. The International Astronomical Union officially named its recommended three-letter abbreviation, “Ari”, in 1922.
The location of the vernal equinox (one of two points where the ecliptic and celestial equator intersect – specifically the intersection when the Sun crosses the celestial equator from south to north) is also called The First Point of Aries, which is named for the constellation. This is because the vernal equinox was located within Aries over two millennia ago. Hipparchus defined this point in 130 BC. Due to the precession of the equinoxes, in which Earth’s axial tilt slowly changes orientation with regards to the background stars, Pisces is now the First Point of Aries, and Aquarius will be the location of the vernal equinox by approximately 2600 AD. The Sun now appears in Aries from late April through mid-May.
History and Mythology
The constellation dates back as far as 1350 to 1000 BC, although Aries was not truly accepted as a constellation until classical times. Aries is the Latin for ram, and the constellation has been depicted as a ram since the time of the Babylonians.
The constellation was not always recognised as a ram. The people of the Marshall Islands saw Aries as a porpoise, and in China the constellation was depicted as twin inspectors. It is not a surprise that Aries was not always thought of as a Ram, as the constellation is relatively faint and indistinct, with only four bright stars. They are the second and third magnitude stars Hamal and Sheratan (Alpha Arietis and Beta Arietis), and the fourth magnitude stars Mesarthim (Gamma Arietis) and 41 Arietis.
Aries featured in Egyptian astronomy and mythology as well. Linked to the Egyptian god Amon-Ra, Aries was shown as a man with a ram’s head. The constellation was the location of the vernal equinox at the time, so the Egyptians dubbed Aries “Indicator of the Reborn Sun”.
In ancient Greek mythology, Aries is the ram which rescues the son and daughter of King Athamas, when his jealous second wife tries to kill them. Although the daughter, Helle, dies during the rescue, the son Phrixus lives and sacrifices the ram to Zeus. The fleece is hung in a sacred place and becomes known as the Golden Fleece.
Though Medieval Muslim astronomers traditionally depicted Aries as a ram, some also showed the constellation as a four legged animal with antlers rather than horns. Persian astronomer Al-Sufi described Aries as a ram looking back as it ran.
Prominent Features in Aries
Stars and Planetary Systems
There are three prominent stars in Aries often used for navigation, and which form an asterism. German celestial cartographer Johann Bayer listed them as Alpha, Beta, and Gamma Arietis. 41 Arietis is a fourth star within the asterism, though being above the fourth magnitude, it is not as prominent as the others.
- α Arietis is the brightest star in Aries. It is also called Hamal after the Arabic word for “lamb” or “head of the ram”. It is an orange, giant star lying 66 light-years from Earth.
- β Arietis is also known as Sheratan, derived from the Arabic word for “the two signs” which alludes to the positions of Beta and Gamma Arietis in signalling the vernal equinox. Beta Arietis is a blue-white star which lies 59 light-years from Earth. Sheratan is a spectroscopic binary star. This means that astronomers only know it has a companion star through analysing the spectra of the star.
- γ Arietis is also called Mesarthim. The meaning of the Mesarthim is conflicting, as it may come from the Arabic word meaning “pair” or the Arabic for “fat ram”. It may also be derived from the Sanskrit for “first star of Aries”. Mesarthim is part of a binary star system which lies 164 light-years from Earth. The scientist Robert Hooke discovered that the star was a double in 1664, marking it as one of the earliest telescopic discoveries of its kind.
There are several other double stars housed in the constellation.
- ε Arietis (Epsilon Arietis) is a binary star system located 290 light years from Earth.
- λ Arietis (Lambda Arietis) is a system consisting of a white primary star and a yellow secondary star, 129 light years from Earth.
- π Arietis (Pi Arietis) is a system approximately 776 light years from Earth, containing of a blue-white primary star and a white secondary star.
A number of other stars in Aries are visible to the naked eye, including:
- δ Arietis (Delta Arietis), also called Boteïn, a magnitude 4.35 star located 170 light-years away.
- ζ Arietis (Zeta Arietis) is a magnitude 4.89 star located 263 light-years away.
- 39 Arietis (Lilii Borea) lies 172 light-years from Earth and is magnitude 4.51.
- 35 Arietis is a faint magnitude 4.55 star, located 343 light-years away.
- 41 Arietis, which is named c Arietis and/or Nair al Butain, lies 165 light-years away and is a magnitude 3.63 star.
- Although 53 Arietis is relatively dimmer at magnitude 6.09, it is interesting because it is a runaway star which most likely originates from the Orion Nebula; having been kicked out during a supernova event. It lies 815 light years from Earth.
- The closest star to Earth in Aries is called Teegarden’s Star. It is the 24th closest star to Earth overall. It is magnitude 15.14 – way beyond what is visible to the naked eye.
- Aries also contains a number of variable stars including R Arietis, U Arietis, T Arietis, and SX Arietis. R Arietis varies from magnitude 13.7 to 7.4 over a period of 186.8 days, lying 4080 light years from Earth. U Arietis has a period of 371.1 days, varying from magnitude 15.5 to 7.2. T Arietis has a period of 317 days during which it ranges from magnitude 11.3 to 7.5, and it is located 1630 light years away from Earth.
- Several of the stars in the constellation are host to exoplanets. Star HIP 14810 is orbited by three giant planets. HD 12661, like HIP 14810, is a star only slightly bigger than our own Sun and is host to two planets which are both slighter larger than Jupiter. HD 20367 is much the same size as the Sun, with one planet – again, slighter bigger than Jupiter – orbiting the star.
Deep Sky Objects
Paired with a good telescope, Aries has a number of deep sky gems worth viewing, and a view which are just outside the amateur’s reach as well.
- NGC 772 is a spiral galaxy with a surface magnitude of 13.6. It is a wonderful target even for amateur astronomers as it is relatively bright – even through a small telescope – and shows nebulosity. NGC 772 is an unbarred spiral galaxy, with a prominent bulge and tightly wound spiral arms. The galaxy is 240 000 light years across and lies 114 million light years from Earth. The main arm is a rich star forming region. NGC 772 has small companion galaxy: NGC 770. It lies 113 000 light years from NGC 772.
- NGC 673 is also a spiral galaxy in Aries. Unlike the unbarred NGC 772, NGC 673 is a weakly barred galaxy with a faint bulge and loosely wound arms. The galaxy is 171 000 light-years in diameter, and located 235 million light years away.
- NGC 678 is an edge on spiral galaxy with a diameter of 171 000 light years. It lies only 200 000 light years apart from elliptical galaxy NGC 680 which measures 72 000 light years in diameter. Both galaxies lie about 130 million light-years from Earth, and both galaxies also have bright cores, though NGC 678 is set apart with its more prominent dust lane.
- NGC 691 is a spiral galaxy with several arms and a bright core, but a low surface brightness. It is 126,000 light-years across, and situated 124 million light-years away.
- Aries is home to a group of eight galaxies of which NGC 877 is the brightest. It lies 178 million light years from Earth and has a diameter of 124 000 light years. The group also includes galaxies NGC 870, NGC 871, and NGC 876. NGC 877 is connected to NGC 876 by a stream of dust and gas. NGC 876 lies 103 000 light years from its companion’s core, and the pair are interacting gravitationally. The galaxies NGC 935 and IC 1801 are also a gravitationally interacting pair in Aries called Arp 276.
- Elliptical galaxy NGC 821 has a diameter of 61,000 light-years and it is 80 million light-years from Earth. It is of interest to astronomers because it shows an early spiral structure not common to elliptical galaxies. Another interesting galaxy in Aries is Segue 2; a dwarf galaxy which is also a satellite galaxy of the Milky Way, and which may have formed around the epoch of reionization.
Meteor showers are usually a bright and beautiful treat of the night sky. Aries is home to one of the strongest day time showers – The daytime Arietid meteor shower.
- The Arietids lasts from 22 May through to 2 July. The Arietids are an annual meteor shower associated with the Marsden group of comets, with the asteroid Icarus being a possible parent body of the meteors. The meteor shower is not visible to the naked eye due to the light conditions, though some meteors may be spotted in the early hours just before dawn. It has a maximum zenithal hourly rate of 54 meteors when it peaks on 7 June. They are observed and studied in the radio spectrum thanks to the trail of ionized gas meteors produce. Aries is home to other day time meteor showers including the Daytime Epsilon Arietids and the Northern and Southern Daytime May Arietids.
- The Delta Arietids is a meteor shower associated with the near-Earth asteroid 1990 HA. They occur between 8 December and 2 January, though the highest rates of meteors are visible from 8 to 14 December. The Delta Arietids peak on 9 December. Though the shower has a low peak rate, it sometimes produces bright fireballs.
- The Autumn Arietids last from 7 September to 27 October, peaking on 9 October (though the peak rate is low). Other meteor showers radiating from Aries include The Epsilon Arietids, October Delta Arietids, Daytime May Arietids, Sigma Arietids, Nu Arietids, and the Beta Arietids.
The zodiac is a rich region of the night sky, and Aries constellation is certainly worth exploring on a dark and clear night, telescope at hand.
Clear and steady skies!