The month of March is named for the planet Mars, which stood for the Roman god of war. March was the first month of the year in the early Roman calendar more than 2,500 years ago.

March always marks the return of spring in the northern hemisphere. This year, that will happen at 6:29 a.m. March 20. It’s also called the vernal equinox, further defined by the sun crossing over the celestial equator on an upward path. When the sun crosses back over the celestial equator on a downward path in six months, that’s called the autumnal equinox. Those are the only two days each year when the sun rises due east and sets due west.

Within a few days of both equinoxes are also the only days that are exactly 12 hours long everywhere except for the poles. The reason that the equal nights and days don’t fall exactly on the equinoxes is that the earth orbits in a slight ellipse and not a perfect circle, as was believed for about 1,000 years before Johannes Kepler came along and proved it wasn’t true.

Now that it’s warming up and the days are getting longer, it’s a good time to head outside under the night sky and enjoy and assimilate just a tiny bit of the great beauty that always surrounds us.

Venus will put on a great show toward the end of the month, appearing in both the evening and morning sky, and its huge thin crescent may even be visible without binoculars. Then Mercury makes its best appearance for the year in the evening sky, and Jupiter gets closer and higher and brighter approaching its opposition in early April. The best occultation of the year will occur on March 4, but unfortunately the Northeast will be the only part of the country that won’t be able to see any part of this great occultation of the moon and Aldebaran in Taurus.

There will be a line cutting through Hartford, Connecticut, and the northern United States. Along it will be a very rare graze occultation, meaning the star Aldebaran will blink on and off several times as its light gets covered and uncovered by mountains and the valleys on the moon along the edge of its northern limb. This is the same cause for Bailey’s beads, which happens right before and right after totality during a total solar eclipse, like the one the entire country will enjoy Aug. 21.

Venus was at its best and brightest in five years during the middle of last month. Now you can watch its dramatic decline from 32 degrees high in the western sky at sunset to 0 degrees just over three weeks later as it disappears below our western horizon. It will reach inferior conjunction on March 25. That’s the closest it gets to Earth for each cycle of 1.6 years. Its crescent will be very thin and huge, and you may even be able to see it without binoculars or a telescope that evening and a few evenings before that. Its huge crescent will cover almost one minute of arc of the sky, or just 30 times less than the full moon covers at half a degree or 30 minutes of arc.

Look for something even more unusual this time. Since Venus will pass more than 8 degrees north of the sun during this inferior conjunction, which is much farther north of the sun than it usually gets at this time, you will actually be able to see this planet in the evening sky after sunset and the morning sky before sunrise. This will happen for more than a week around the 25th. Start looking for Venus before sunrise as early as March 15. By March 22, Venus sets 30 minutes after the sun but already rises 33 minutes before the sun the next morning. That time will extend to 40 minutes by the 25th. Try to look at Venus with a telescope during this time and look for wispy cusp extensions of its thin crescent. These are caused by the back scattering of sunlight through its thick atmosphere. The only time this is more dramatic would be right before or after a transit of Venus, which happens eight years apart, then not again for another 121.5 years. I saw the last two transits in June 2004 and June 2012.

Venus is a very strange planet. Its surface temperature is almost 900 degrees, hot enough to melt metals like lead and tin. At its surface, its air pressure is almost 100 times more than ours. At about 40 miles above the surface of Venus, its pressure equals ours on the surface of Earth. The enormous pressure on Venus is similar to being 3,000 feet under the ocean on Earth. Its atmosphere has high levels of carbon dioxide because infrared light from the sun can get through its atmosphere, but then some can no longer escape back out into space. That is what makes a greenhouse on Earth work, but there’s way too much carbon dioxide in the atmosphere of Venus. Its atmosphere is over 96 percent carbon dioxide and only about 3 percent nitrogen. Ours is about 78 percent nitrogen and 21 percent oxygen, and a few other trace elements. We only have about 400 parts per million of carbon dioxide.

One year on Venus equals 225 days, but a single day is equal to 243 Earth days. Venus also spins in the opposite direction of most planets, so the sun rises in the west each morning, but that’s not often. Even more interesting, Venus makes 13 trips around the sun every eight years and reaches an inferior conjunction with Earth five times in those eight years, or every 1.6 years. That is the famous golden ratio, also known as phi. The other three numbers are all Fibonacci numbers. That is the famous sequence that you get by simply adding the two previous numbers to get the next number. The ratio of any two of these numbers is 1.618. Many things in nature grow this way, including sunflowers, galaxies, and twigs on a branch.

Look for Mars about 15 degrees up and to the left of Venus. Mars continues to fade slowly but is still brighter than any nearby stars. It sets a little earlier each night, but we won’t lose it completely until early July. After that, it will just show up again in the morning sky. It’s about as far from Earth now as it can get. Saturn continues to rise a little earlier each night in Scorpius, now a little after 1 a.m. It won’t reach opposition until the middle of June, just before summer starts.

MARCH HIGHLIGHTS

March 1: Mars and Venus are near the moon this evening.

March 4: The moon in the Hyades in Taurus occults Aldebaran tonight.

March 5: First-quarter moon is at 6:33 a.m.

March 12: Full moon is at 10:55 a.m. This is also called the Worm, Crow, Sap or Lenten Moon. Daylight saving time starts at 2 a.m. Sunday.

March 13: In 1781, William Herschel discovered the planet Uranus.

March 14: Albert Einstein was born on this day in the year 1879. Jupiter, Spica and the waning gibbous moon form a nice triangle in the east around 10 p.m.

March 20: Last-quarter moon is at 11:59 a.m. Saturn will be near the moon this morning.

March 22: On this day in 1997, Comet Hale-Bopp made its closest approach to Earth.

March 25: Venus reaches inferior conjunction with the sun tonight.

March 27: New moon is at 10:58 p.m.

March 29: Mars is near the slender waxing crescent moon this evening and the next.

Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England.

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