Physics Photo of the Week
Smiley Moon and upright Moon
Graduate Joel Barto, a reader of Physics Photo of the Week, recently
asked the question: "When we view the crescent Moon near sunrise or
sunset, why does it sometimes appear as a 'smile' and other times it
appears upright, like a parenthesis?" My first thought was: "That
is a very good question - I had never thought about that before nor
noticed it before!" Thank you, Joel, for inspiring a Physics
I had to wait until 2 equinox's had passed in order to obtain pictures
that Joel had suggested. Both these photos show the crescent Moon
in the pre-dawn sky. The top photo, made this past September,
near the fall equinox clearly shows the "smiley" Moon. The bottom
photo from this past April, close to the Spring equinox, shows the Moon
more erect resembling a parenthesis.
The Moon and other planets all orbit the Sun in roughly the same plane
- the ecliptic
plane - which is the plane of the Earth's orbit. The ecliptic
plane is tilted 23 1/2 degrees from the celestial equator. This
is another way of stating that the Earth's equator is tilted 23 1/2
degrees from its orbit. When plotted on the sky, the ecliptic
crosses the celestial equator at two points - the equinoxes. The
drawing below shows the Sun at both the fall equinox and the spring
equinox. The ecliptic is oriented about 23 degrees above the
equator at the September equinox and about 23 degrees below the equator
at the March equinox. Because the Moon is close to the ecliptic
throughout its orbit, the crescent Moon in the eastern sky before
sunrise shows the "smiley" or upright crescent.
When the crescent Moon appears at sunset,
it "smiles" at the spring equinox and is upright at the
fall equinox reversing the orientations.
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to firstname.lastname@example.org.
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