Expert: Tom Whiting - 2/4/2008

Question
I was in Costa Rica mid january 08 and saw a quarter moon in the evening in the western sky. It was like a horizontal saucer that you could fill with water.
All calendars show moon phases in a vertical position.
Is the quarter moon also like an upside down saucer sometimes? Is there a site I can see real pictures of the phases as they actually appear?

Answer
Hi Trevor,
Yes, you are talking about the angle that the quarter  (or crescent) moon makes with the closest horizon, and relative to your direction of gravity, thus the "saucer filled with water" concept.  This is all dependent on.... where on the ecliptic (plane of the sun-Earth) the quarter or crescent moon is located.
Sometimes the ecliptic at quarter moon makes a very steep angle
to the horizon, (like December and January)  so the quarter moon appears as you saw.  Other times of the year, the ecliptic makes a very shallow angle to the horizon so the quarter moon appears nearer the vertical position.  So it varies with the month of the year, and whether you're seeing a last quarter around midnight rising in the east, or a first quarter around midnight, setting in the west.

Your first quarter moon in January was in Pisces near the Vernal Equinox so the ecliptic makes it's steepest angle to the western horizon at that time.  Check it out again, the first quarter moon setting on June 9-10-11 time frame, and you'll see a first quarter moon setting in the west that is nearly vertical. (The opposite of what you saw).  Your location doesn't matter, as that
only "moves" the lunar disk slightly north or south in our night
sky, due to parallax.
Calendars only print the symbol for quarter phase by convention,
and the way the quarter moon is viewed most of the time in our sky,
NOT necessarily at moonset time.  Most of the time when viewed in
our night sky, like high in the south, the quarter moon is in the near-vertical position, thus I guess calendar makers picked the most
convenient, and most seen, symbol.  We already know the moonset
appearance will vary by the angle of the ecliptic to the horizon,
from month to month, as you reported.
The best program I know to see the relative position of the moon
setting to the horizon is the "night sky" view of Google Earth...so
if you have Google Earth, punch up the night sky component, and
that program accurately displays the moon's setting appearance relative to your horizon.
Hope all this helps,
Clear skies,
Tom Whiting
Erie, PA