Hamal – also known as Alpha Arietis – shines as the brightest star in the constellation Aries the Ram. This star and two others – Sheratan and Mesarthim – make up the Head of the Ram. Aries is small. But the compact pattern of these three stars makes Aries relatively easy to find. As seen from mid-northern latitudes, Hamal lights up the eastern sky on autumn evenings, shines high in the southern sky on winter evenings, and sits in the west on early spring evenings. Hamal disappears from the night sky in April, then returns to the eastern sky before sunrise by late spring or early summer, to begin another cycle of visibility.
It’s fun to spot Hamal and its brother stars in the night sky. But this star also has a profound significance in the history of astronomy.
If you could see the stars in daytime, you’d see the sun and Hamal in conjunction (lined up with one another, due north and south in right ascension) on or near April 24. What this means is that – when the sun appears due south at noon – Hamal is due south, too, though lost in the glare of the noonday sun.
April 24 – the date of Hamal’s conjunction with the sun – is a little more than one month after the March equinox, which always takes place around March 20. This is the Northern Hemisphere’s spring equinox, and it’s a time of renewal throughout the northern half of Earth. So of course this time of year had significance to our ancestors, who were much more aware than we are of their dependence on the land and sky.
What is the relationship of Hamal to the March equinox? If you could backtrack some 2,500 years, you’d find the annual conjunction of the sun and Hamal happening on the March equinox. In fact, if you could backtrack 2,200 years, we’d find the March equinox sun in conjunction with the star Sheratan. So you see that the location of the sun at the March equinox sun drifts in front of the stars. It moves westward in front of the backdrop constellations by about one degree (two sun diameters) every 72 years. This drifting is due to a well-known motion of Earth called precession, or sometimes the precession of the equinoxes.
The March equinox sun shone in front of the constellation Aries from about 2,000 to 100 BCE. At present, the sun shines in front of the constellation Pisces on the March equinox. Even so, many people pay homage to the Ram and still refer to the March equinox point as the First Point of Aries.
Bottom line: The star Hamal, also known as Alpha Arietis, is the brightest star in Aries the Ram. Thousands of years ago, the sun was in conjunction – or aligned north and south – with this star at the time of the March equinox. Nowadays, we see the sun in front of Pisces at the time of the March equinox. But people still refer to the sun’s location at the equinox as the First Point in Aries.
Sky chart of the constellation Aries the Ram
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.