In this month of the spring equinox, the Moon will point us toward some noteworthy celestial objects and astronomical phenomena.
The lunar crescent between the Pleiades and Hyades
The crescent Moon will first wander between the Pleiades and Hyades star clusters on the evening of March 8, a fleeting encounter that will provide nice opportunities for astrophotographers and anyone looking up with the naked eye or binoculars. As soon as twilight sets in, the trio appears very high in the southwest, then it slowly sinks toward the west-northwestern horizon with each passing hour and vanishes there around midnight.
The Hyades open cluster is the group of stars we see near the brilliant orange star Aldebaran, in the constellation Taurus. Located 150 light-years away, it is the second closest cluster to us; in fact, it is 70 light-years further away than the sparser Ursa Major open star cluster, which includes six of the stars that draw the famous Big Dipper asterism. To date, about 1,800 stars have been recorded in the Hyades, but most are too faint to be visible to the naked eye. Despite appearances, Aldebaran is not part of this cluster, since it sits roughly halfway between the Earth and the Hyades and doesn’t share the same origin. The stars that make up the Hyades cluster were all born at about the same time (on an astronomical timescale), some 680 million years ago. Imagine the Earth at that time: Life on our planet was still primitive, composed mainly of plankton, algae, amoebae and the first sponges. The Earth had also undergone two major ice ages, which occurred during a geologic period aptly named Cryogenian. As recently as 2019, an enormous stream was discovered around the Hyades, stretching 600 light-years across, consisting of stars that were still part of the cluster a few hundred million years ago and which are slowly drifting away.
About 440 light-years distant, the Pleiades open star cluster is somewhat further away than the Hyades, but much younger at 112 million years old. It’s also easier to see with the naked eye, since the stars that form the Pleiades are concentrated in a smaller area of sky (given its greater distance from us). To date, around 1,800 stars have been identified as members of the Pleiades; once again, only seven or eight are visible to the naked eye, and a few dozen can be viewed through binoculars or a small telescope. When the stars of this cluster were forming, Earth was just entering the Cretaceous period: Dinosaurs were in their golden age and the still-insignificant mammals only played a secondary role in the ecosystems.
Over the next few weeks, the Moon will occult two bright stars observable from southern Quebec. In the early evening of March 15, the waxing gibbous Moon will pass in front of Eta Leonis (a supergiant located about 1,300 light-years from us), effectively hiding the star for slightly more than an hour. In fact, Eta Leonis is a binary system formed by two extremely luminous massive stars near the end of their lives. At the distance they’re located, these two stars are separated by a mere 0.1 to 0.4 arc seconds, depending on the year—too close to be visually separated in a small amateur telescope, but the right equipment pointed at them during a lunar occultation could reveal their binary nature. As seen from Montreal, the disappearance takes place at 7:57:04 p.m. EDT at the Moon’s dark limb (38° high in the east-southeast); the star reappears at 9:04:20 p.m. from behind our satellite’s illuminated edge (48° high). The exact timing of the star’s disappearance and reappearance may vary by several seconds, since it depends on the viewer’s precise geographic location.
Next to be occulted by the waxing gibbous Moon will be the Gamma Virginis star, during the night of March 18 to 19. Gamma Virginis is significantly less luminous than Eta Leonis, but at “only” 39 light-years from us, it is also much closer and therefore appears somewhat brighter. It, too, is a binary system whose two stars are just barely more massive and hotter than the Sun. Thanks to detailed observations of their orbit, they are two of the few stars whose masses can be accurately measured using Kepler’s laws: Each star has about 1.45 times the mass of the Sun. Their angular separation varies between 1 and 7 arc seconds depending on the year, and it is easy to view them separately in a small amateur telescope. As seen from Montreal, the disappearance takes place at around 12:44:09 a.m. EDT behind the Moon’s illuminated edge (41° high in the south-southeast); the star reappears at around 1:47:12 a.m. at our satellite’s dark limb (43° high in the south).
The Moon continues its journey around the Earth, wandering through the constellations of the zodiac as its phase slowly wanes with each passing day. On the morning of March 28, shortly after 6 a.m., the lunar crescent lies below brilliant Venus, very low on the southeastern horizon; the horizon must be perfectly clear in this direction to spot the Moon. Carefully scan the sky near the Morning Star to make out two other dots of light: Saturn, which lies just below Venus, and the slightly fainter Mars, shining 5 degrees to the right of its two sisters.
Finally, remember that this year’s spring equinox takes place on March 20 at 11:33 a.m. EDT. The term equinox comes from the Latin aequinoctium, meaning “equal night,” alluding to the fact that the two annual equinoxes are when the length of the day is equal to that of the night. In fact, the equinox is more precisely defined as one of the two moments in the year when the Earth’s equator is directly aligned with the Sun; in other words, there is a spot on Earth along the equator where the Sun is exactly at the zenith (remember that the Earth’s equatorial plane is inclined by approximately 23.5 degrees from its orbital plane). If the Earth lacked an atmosphere and the Sun were nothing more than a tiny dot in the sky, then day and night would indeed be of equal length at the exact time of the equinox. But in reality, the Sun appears as a disc, with its top edge still visible a few minutes after its centre has dipped below the horizon. Moreover, because Earth’s atmosphere bends light rays slightly upwards, the Sun’s disc is visible even if it is in fact just below the horizon, geometrically speaking. This is why at Montreal’s latitude (45.5 N), the day is about 10 minutes longer than the night at the equinox; the true “equiluxes” occur about three days before the spring equinox (in March) and three days after the autumn equinox (in September).