See also

+ Sky Map for March 2010

Springtime, You Say?

Nathalie Martimbeau, astronomer, Montréal Planetarium

It’s hard to believe that March is already here, and it’s still so cold! Yet this is the month of the spring equinox. So let’s take a look at the phenomenon we call the seasons.

Spring arrives this month!

The March equinox, which marks the beginning of spring in the Northern Hemisphere, takes place this year on the 20th at 13:32 EDT. The word “equinox” comes from the Latin meaning “equal night.” It refers to the two moments in the year when day and night are the same length. But what is an equinox exactly? And what brings about the changing seasons? The answer can be found in the tilt of the Earth’s axis.

Imagine a line passing through the Earth from the North Pole to the South Pole. This polar axis is not vertical compared to the plane of Earth’s orbit around the Sun: It’s tilted 23 ½ degrees, and it always points toward the same direction in space. When the Earth arrives at the point in its orbit where the Northern Hemisphere is most tilted toward the Sun, it’s the summer solstice, which marks the beginning of summer. Six months later, the Earth is at the opposite point in its orbit and the Northern Hemisphere is tilted away from the Sun: that marks the winter solstice and the start of the winter season. At the equinoxes, the axis of rotation is perpendicular to the Sun, which appears exactly above the equator and illuminates both hemispheres — north and south — equally. This moment signals the beginning of spring or autumn, depending on the case.

The Earth’s path around the Sun and the tilt of its rotational axis. The angle of the Sun’s rays on the Northern and Southern Hemispheres changes according to the Earth’s orbital position, which produces the phenomenon of seasonal change. (Illustration: Sophie DesRosiers, Planétarium de Montréal)

The planets, and Moon …

The planet Mars, presently located in Cancer, is well up in the southeast as evening begins. As it recedes from Earth, its brightness diminishes: Right now it’s comparable to the bright star Capella in the constellation Auriga, or Rigel in Orion. For several weeks, Mars has been moving westward among the stars: This retrograde movement ends on March 11, when the Red Planet will appear to stop momentarily before resuming its direct, eastward path. On the evening of March 24, a gibbous Moon will be to the right of Mars, and the following evening it will be to the left. On March 30, the Red Planet is at aphelion — its greatest distance from the Sun.

Saturn is above the eastern horizon as evening begins, and remains visible throughout the night. Though the planet is currently in Virgo, it is easier to locate by looking beneath Leo, the lion. On the evening of March 1, look toward the eastern horizon and you’ll see a gibbous Moon to the right of Saturn. This scene will be repeated on the evening of March 30, when the Moon will be nearly full. Saturn will be in opposition on March 21, which means the distance separating the Earth from the ringed planet will be at a minimum for the year. Saturn will be its brightest, and through a telescope its apparent diameter will be at its maximum. Unfortunately, Saturn’s rings are only inclined less than 4 degrees right now, so they appear like a thin line against the planet’s disk.

Brilliant Venus can be seen right after sunset, to the left of a thin crescent Moon, on the evening of March 16 — if your western horizon is very clear. The following evening at the same hour, the crescent Moon will be above Venus. Binoculars will add to your appreciation of the view, and astrophotographers will enjoy capturing the beauty of this scene, bathed in the colours of twilight.

… and a few constellations

The constellations of spring are now in full view. Leo is well up in the southeast and is easy to spot : look for a backwards question mark with a right triangle to its left. Regulus, the star at the bottom of the question mark, is hard to miss : it’s one of the brightest stars in this part of the sky. In the southwest, we can still see the constellations of winter: Orion the hunter and his companions, Canis Major and Minor, Gemini the twins, Auriga and Taurus. Sweep the northern horizon with your eyes and you’ll find: Perseus, Casseopeia, Cepheus, Draco and Boötes. Nearly overhead, you’ll see Ursa Major with its brightest stars forming the Big Dipper, though in its current position standing on end, it has certainly emptied its contents on the horizon!

In closing, a quick reminder: It’s Daylight Saving Time again. Our clocks move ahead one hour on the night of March 13 to 14.

Happy observing!