After a long cold winter March has finally arrived, and with it the spring equinox. Also this month, four naked-eye planets are visible: Venus, Mars, Jupiter and Saturn… plus one that’s almost naked-eye, but worth observing nonetheless — Uranus!
A season of change
Though winter isn’t quite over yet, March signals the gradual transition to spring: The Sun climbs above the celestial equator into the northern sky, days grow longer and warmer, and by mid-month the springtime constellations occupy centre stage. March is also when we set clocks ahead one hour as we shift to Daylight Time.
“Springing ahead” gives us the impression that the days have suddenly gotten longer: Instead of setting at 6:00 P.M., the Sun suddenly sets at 7:00 P.M.! Of course it also rises an hour later, which cancels out any effect; but a careful observation of sunrise and sunset reveals that the days really do get longer at this time of year, and rapidly so. Throughout the month, the Sun rises and sets about half-a-degree (one apparent solar diameter) further north along the horizon each day. Due to this northward movement, the Sun takes a higher, longer path across the sky, so we gain daylight hours substantially in March: In fact, the gain from beginning to end of the month totals about one hour and forty minutes of extra sunlight.
Here are a few more interesting springtime facts: This year, the Sun will cross the celestial equator, which is the projection of Earth’s equator onto the sky, on March 20 at 6:45 P.M. EDT. On the equinox, the Sun rises due East and sets due west; and the number of daylight and nighttime hours are equal, hence the term – equinox. Also, during springtime twilight at mid-northern latitudes, the ecliptic – the orbital plane of the planets – intersects the western horizon at a much steeper angle (about 68 degrees) than it does at dawn (22 degrees). As a result, evening planets are stacked almost vertically above one another: This amounts to a rapidly increasing altitude, from planet to planet, above the horizon, which makes them easier to observe in the deepening twilight. Take Uranus for example…
A faint green giant
Uranus isn’t normally thought of as a naked-eye planet, though in reality it is. The ice giant orbits roughly half-way between the Sun and the distant dwarf planet Pluto, about 3 billion kilometers from Earth. Though it measures about four times Earth’s diameter, because of its distance Uranus shines at magnitude 5.8; however, the limiting magnitude for naked-eye visibility is considered around magnitude 7, depending on factors such as sky darkness, eye sensitivity, and the object’s altitude above the horizon. But all factors aside, a good pair of binoculars will bring the planet within anyone’s reach. So if you haven’t seen Uranus before, this month offers a golden opportunity, with Venus as your guide.
On March 1, dazzling Venus is impossible to miss shining brilliantly above the western horizon. About an hour after sunset, center Venus in your binoculars and you’ll see the Red Planet Mars shining about three-and-a-half degrees to the lower right. Now look to the upper left, about the same distance from Venus as Mars, and you’ll find a faint aqua-green dot glowing steadily: That’s Uranus. All three objects will be evenly spaced in the same 8-degree binocular field of view. During the following evenings, Venus climbs ever higher and gradually overtakes Uranus: On March 4, the dazzling white planet sits just a quarter of a degree above the faint green giant. Then, over the coming evenings, its Mars’ turn to overtake Uranus: On March 11, the Red Planet will sit about a third of a degree above the ice giant. The contrast between Mars’ orange hue and the icy aqua-green of Uranus should be striking.
A thin lunar crescent will be to the lower left of Mars on the evening of March 21 and to the lower left of Venus on March 22, but by then Uranus will be too close to the horizon to observe, even with binoculars. If you want to see the faint aqua giant you’ll have to do it during the first half of the month – the sooner the better.
Jupiter and Saturn
Two more giant planets are also visible in March. Jupiter is well up in the south-eastern sky at twilight, and is retrograding among the stars of Cancer. The gas giant culminates by mid-evening, which is the best time to observe it, along with its contrasting cloud bands and Galilean satellites. Meanwhile, Saturn rises around 2:00 A.M. at the beginning of the month, and around midnight toward month’s end. By mid-March, the ringed giant culminates low in the south just before dawn. Though it is not currently well placed for observing, Saturn’s rings are tilted 25 degrees toward Earth, which makes the planet an interesting target nonetheless.
The waxing gibbous moon appears next to Jupiter on the evening of March 2, and again on the evening of March 29; the waning gibbous moon will appear just above Saturn during the pre-dawn hours of March 12.
Clear skies to all!
Planets visible to the naked eye