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Because of Ever-Returning Spring!
Why does the waxing crescent Moon smile in the month of March? It could be said that the Moon always smiles when it's reborn after the new moon at every month of the year. However, it's only in late winter and early spring that the moonlit smile appears horizontally over the western horizon, and looks like a "lunar boat" floating over the horizon.
The young lunar crescent closest to the spring equinox always appears boat-shaped. At other times of the year - most notably in late summer and early autumn - the "Cheshire cat" smile of the young crescent appears most askew, looking like a backwards "C". Why does the "lunar boat" appear near the time of the spring equinox and the backwards "C" around the time of a Northern Hemisphere autumn equinox? In short, it's because of the changing tilt of the ecliptic.
Tilt of Ecliptic Highest on Late Winter/Early Spring Evenings
The plane of the Earth's orbit around the Sun is called the ecliptic. The ecliptic as shown on sky charts is actually a projection of the Earth's orbital plane onto the stellar sphere. Because the Moon orbits the Earth, and the planets orbit the Sun, on nearly the same plane that the Earth orbits the Sun, you'll always find the Moon and planets on or close to the ecliptic.
However, the tilt of the ecliptic in the early evening sky doesn't remain constant throughout the year. The ecliptic swings highest in the evening sky on late winter and early spring evenings. In other words, when you're looking westward on a late winter/early spring evening, the ecliptic tilts most steeply to the horizon, intersecting it at almost a right angle.
Keep in mind that the lunar crescent always runs perpendicularly relative to the plane of the ecliptic. So on late winter/early spring evenings, the thin lunar crescent comes closest to being horizonal because that's when the ecliptic comes closest to becoming vertical.
Evening Crescent of March 17, 2010
If, indeed, a picture is worth a thousand words, the sky chart at the upper right-hand side of this page illustrates why the thin waxing crescent Moon appears horizontally over the horizon on March 17, 2010. Remember to look for the Moon at dusk and early evening, for the thin lunar crescent will follow the Sun beneath the horizon about 2 hours after sunset on March 17. You can also look for an even younger crescent on the day before - on March 16, 2010 - although the moon will set only about one hour after the Sun on this date. For an almanac that gives the sunset and moonset times for your sky, check out the links on EarthSky's almanac page. While you're at it, use binoculars to see the earthshine softly illuminating the night side of the Moon.
Shortly after sunset on March 16 and March 17, 2010, the thin waxing crescent Moon appears pretty much above the sunset point on the horizon. With the Sun lurking almost directly below the Moon, sunlight illuminates the underneath part the lunar disk. The crescent goes from left to right rather than up and down.
Fast Forward to Evening Crescent of September 11, 2010
At the other end of the year - on late summer/early autumn evenings - the ecliptic makes the shallowest angle with the horizon. Therefore the waxing crescent stands off to the left of the sunset point on the horizon, and sunlight falls on the right-hand side of the lunar disk. That's why the lunar crescent appears more up and down, and paints a backwards "C" in the early evening sky at this time of year.
My Thanks to John Walker's Your Sky
I wish to thank John Walker's YourSky Interactive Planetarium for enabling me to provide the sky charts in this article. The charts are for mid-temperate latitudes in the Northern Hemisphere. Incidentally, the celestial equator is the Earth's equatorial plane projected onto the celestial or stellar sphere. Unlike the ecliptic, the celestial equator remains fixed in the respect that it interesects the horizon at the same angle all year long.
Ecliptic Highest Up:
Sunset, Spring Equinox Noon, Summer Solstice Sunrise, Autumn Equinox Midnight, Winter Solstice
copyright 2010 by Bruce McClure
February 2010 Feature * April 2010 Feature
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