Thursday, April 10, 2008


This week, let’s watch the waxing Moon do-sa-do with planets and other objects as it sashays along the ecliptic. The waxing Moon is a growing moon. Each night, more of its face is illuminated by the Sun, from our changing vantage point on Earth. In actuality, half of the Moon’s sphere is illuminated by the Sun at all times. However, because of Earth’s position at the center of the Moon’s month-long promenade, we only ever see part of the Moon’s Sun-illuminated hemisphere, except during Full Moon and New Moon.

“Swing and circle, wheel around…”

The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, the Moon, and the planets all lie in roughly the same plane as they orbit the Sun, the ecliptic can also be said to represent the plane of the solar system. As a result, we see the Moon and the planets sticking close to the ecliptic as they swing across the sky.

“Promenade family, chain down the line…”

Face south for all these observations, for which you will need no equipment but your eyes.

On Friday evening, April 11, the Moon’s face is 39% illuminated, as seen from Earth. Just to the left of the Moon is the bright planet Mars. Notice its reddish or coppery color; this distinct coloration is from its soil, which is rich in iron oxides. Mars is currently spending some quality time in the constellation of Gemini the Twins, and the Moon joins it there for the entire evening, until they both set in the west after midnight.

Slightly above the Moon and Mars are a pair of stars, the two brightest in Gemini: Pollux on the left and Castor on the right. In Greek mythology, Pollux and Castor were inseparable half-brothers with the same mother. When the mortal Castor was killed in a dispute, immortal Pollux was inconsolable. His father, supreme ruler Zeus, took pity on him and allowed him to live in the sky with his brother for half of the year.

Amazingly, the slightly dimmer Castor is actually a six-star system! With a telescope at high magnification, I have seen the three main component stars, but the three companion stars are too close to their partners to be visible in any telescope.

“Acey deucey, boys run right…”

On Saturday, April 12, the Moon reaches its First Quarter phase, when it is 50% illuminated as seen from Earth. This is sometimes referred to as a “half moon.” Note that the Moon is now to the left of Pollux and Castor. Tonight it straddles the boundary between the constellations Gemini the Twins and Cancer the Crab. Precisely-drawn boundaries between constellations were officially designated in 1930 by astronomers of the International Astronomical Union.

“Tag your neighbor, here comes the judge…”

On Sunday evening, April 13, the Moon is 61% illuminated as seen from Earth and is called a gibbous moon. The Moon is gibbous whenever more than half of its face is illuminated, the exception being a Full Moon, when its face is 100% illuminated. Tonight it is more than halfway across Cancer. Hidden in its glare is the Beehive, a wonderful naked-eye star cluster that has been known since antiquity. We will examine the Beehive in greater detail in a few weeks.

Every night at the same time, the Moon is farther to the east than it was the night before. This is because it orbits Earth counterclockwise, as viewed from above the North Pole. From our perspective in the Northern Hemisphere, it travels from right to left--or west to east--over the course of a month.

“Pass to the center, ride the tide…”

On Monday evening, April 14, 71% of the gibbous moon is illuminated, as seen from Earth. The Moon is now in the constellation Leo the Lion. Just to its left is the bright star Regulus, also known as “The Lion’s Heart.” Continuing a little farther to the left, we come to the even brighter Saturn, shining like a golden star. The iconic ringed planet is 750 million miles from Earth.

“Shoot the star, rollaway, weave the ring…”

On Tuesday and Wednesday, April 15 and 16, the Moon continues to travel east through Leo, following the gentle curve of the ecliptic. On the 16th, the Moon will glow--87% illuminated--below Denebola, the bright star that marks Leo the Lion’s tail.

Did you notice that the three constellations the Moon is traveling through this week--Gemini, Cancer, and Leo--are zodiacal constellations? This is because the zodiac is by definition the band of 12 constellations that straddles the ecliptic. As noted earlier, the Moon and the planets hug the ecliptic and, therefore, we see them moving through the zodiacal constellations over time.

“Flip the hourglass, sashay thru!”