When the moon hits your eye like a big pizza pie...
Full Moon at perigee, vernal equinox, planet-spotting.
...that's amore. Is it just me, or is that song always playing on the PA system at Olive Garden?
Big moon tonight
March's full moon is tonight. That's reason enough to glance skyward―I never get tired of looking at the moon―but tonight's full moon is exceptional. For the first time in 18 years, the full moon is taking place within one hour of the moon's perigee, or its closest approach to Earth.
What this means for us Earthbound folk is that the moon will appear fully 14 percent bigger tonight than it does when at its apogee, its orbit's most distant point from Earth. (Any natural or man-made object that proceeds around the Earth in an elliptical orbit―and that's most of them―has a perigee and an apogee.)
The moon's average distance from Earth is about 239,000 miles. Tonight, it will be just 221,000 miles away. It will rise at 6:33pm, just as dusk is setting in, so try to remember to pop outside during or shortly after dinner and look to the east. It's shortly after moonrise, when the moon can be viewed behind the land-based objects that give our brain its frame of reference, that the moon always looks biggest. Tonight, we can use that optical illusion to enhance a full moon that really is bigger than usual!
Tomorrow evening at 7:21pm marks the spring, or vernal, equinox―the official beginning of spring.
What exactly does that mean? I'm glad you asked (and if you didn't, that's okay, too).
For a moment, think about the Earth's orbit around the sun. Now imagine that the oblong circle, or ellipse, of that orbit was a flat disk. If you think about the Earth going around the edge of that disk, you might be tempted to imagine the North Pole at the "top" and the South Pole at the "bottom."
This is actually not the case. The earth's axis is tilted 23.5 degrees off from from the plane of its orbit around the sun. In effect, the Earth is "lopsided" as it travels around the sun…and that's why we have seasons.
Contrary to some beliefs, the seasons have nothing to do with the Earth's closest approach to the sun (perihelion, from Helios, the ancient Greek name for the sun god) or furthest point from it (aphelion). In fact, the Earth is at aphelion in the middle of our summer! The variation in the Earth's distance from the sun during its orbit has only a minuscule effect on temperatures.
The Earth's 23.5 degree tilt means that for most of the year, either the Northern or Southern hemisphere is getting the lion's share of the sun's light. Depending on where it sits in its orbit, the Earth is tilting one hemisphere or the other towards the sun. The only two times during the year when that's not true are the equinoxes―one in the spring, and one in the fall. At those two points in the Earth's orbit, both hemispheres are receiving the same amount of sunlight. After today, our days begin getting longer until the summer solstice in June, when the Northern hemisphere is most heavily tilted towards the sun.
Of course, the opposite is true for the Southern hemisphere. For those of you with friends or relatives in, say, Argentina, South Africa, or Australia, today is actually the autumnal equinox.
Saturn and Mercury
This is also a good time to spot Saturn and Mercury. If you're looking at tonight's full moon, look for Saturn trailing it a little bit to the east. With the naked eye, you'll see a bright "star" a few handspans to the left of the Moon. Unlike a star, though, Saturn will not appear to "twinkle." If you're blessed with keen eyes and a good pair of binoculars, you may even be able to perceive Saturn's oblong shape, created by its famous rings. To see the rings in any detail at all, you'll need a telescope. Saturn rises at 8:21pm tonight and will rise earlier each day. On April 1, it will rise at 7:25pm.
This is also a good time to spot Mercury, which is often one of the trickier planets to see because it's small and often gets lost in the Sun's glare. The best day to see Mercury is this coming Tuesday, March 22, about half an hour after the sun goes down. Keep an eye on the sun as it sets. Note the spot on the horizon where the sun dips below it.
Starting about 15 minutes after the sun sets, start looking for two bright points of light. The brighter one will be Jupiter, the largest planet in our solar system. It's on the opposite side of the Sun from us right now.
Mercury will be about 12 degrees above Jupiter, or roughly the width of your fist at arm's length, measuring from your thumb to your pinky. It's in a crescent phase right now, so you may need binoculars or a keen-eyed friend to help you spot it.