Tonight the Moon is just two days away from full and will blot out the lesser stars until nearly dawn. With the naked eye you can clearly see the Moon has two types of surfaces – the darker maria and the lighter highlands. The maria (mare is singular) have been given colorful names in Latin and are generally referred to as “seas” although they are merely dry lava flows. The circular sea in the middle of the northern hemisphere is Mare Serenitatis (sea of serenity), just to the southwest of Mare Serenitatis is Mare Tranquillitatis (sea of tranquility). To the east of Mare Serenitatis is the Mare Imbrium (sea of rains) and to the southeast of that is Oceanus Procellarum (ocean of storms). More maria and some craters can be seen with binoculars. The best time to look for maria is at the full Moon, but craters are best located when they are near the terminator – the line between lit and unlit portions of the lunar surface.
On the published sky chart you can often see a solid line, this is the ecliptic, the path the Sun follows across the sky each year. (For those of you who want to be technical it’s the Earth’s orbit projected on the celestial sphere.) All of the major planets and the Moon can always be found within a few degrees of this line. The ecliptic runs through thirteen constellations (not twelve!) with familiar names like Aries, Pisces and Aquarius. As you might of guessed by now this band of sky centered on the ecliptic is traditionally called the Zodiac.
Today there is an extra line on the sky chart, the celestial equator, a projection of the Earth’s equator on the sky. The spot where the two lines cross (just to the left of Jupiter tonight) is traditionally called the First Point of Aries even though it’s in Pisces. When the Sun arrives at this spot on March 20 or 21st it’s called the Vernal Equinox – the start of Spring in the northern hemisphere. It’s also the zero point of the astronomical coordinate system. The zero hour line of Right Ascension (equivalent of longitude on Earth) runs from the North Celestial Pole next to Polaris, through the First Point of Aries, and onwards to the South Celestial Pole.
To the eternal annoyance of astronomers the First Point of Aries slowly moves. Very slowly to be sure, but astronomers have to redraw their charts to match the moving coordinates every 50 years. A couple of millennia ago the point really was in Aries and about 6 centuries from now it will be in Aquarius. (The Age of Aquarius!)
Mr. Usher is Vice President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.