Back to the
Ask An Astronomer page.
Hello,
I am Debra Fischer, a grad student at Lick Observatory and UCSC
responding to your question about when (and where) the rising moon
and setting sun would both be visible in the sky.
The full moon is 180 degrees from the Sun, so the zeroth order
answer to your question is that the rising full moon should be
visible as the sun is setting. The situation is complicated a
bit because 1) true full moon doesn't always occur at the
exact time the sun is setting and 2) the moon is moving around
the Earth. So in fact, there is a band that sits on the sunset/
sunrise line around the entire Earth. The position of that band
should rotate on the Earth. (This discussion is best done with
a globe in hand!)
If you have a globe, set it in front of you. Imagine that the
sun is shining from one direction onto the globe, and from the
opposite direction, the moon is shining. If it is summer, the
tilt of the Earth puts the sun high above the equator (but then the
moon is low in the sky!) There is a belt around the Earth where you
can imagine the sunlight and moonlight overlapping. This belt is
not 90 degrees from the equator because of the tilt of the earth.
(Instead, it will be perpendicular to your desktop, if the globe
really is sitting in front of you).
Now, if the moon "stood still", as the Earth turned that band would
sweep out most of the surface area of our planet. But the moon is
orbiting the Earth in the same sense that the planet is rotating.
So the key is to be right on that sunset-sunrise (or alternatively,
sunset-moonrise) band. In fact, you want to be a bit ahead of
that band because you've got to see around the "belly" of teh Earth.
So find a time when the phase of the moon is just a little earlier
that full (technically, waxing gibbous) *at sunset* and you should
see moonrise and sunset in the same sky.
There is a query page that can help you determine the phase of the moon
at various times and locations in the US. It is a web site maintained
by the US Naval Observatory:
http://tycho.usno.navy.mil/srss.html
As to the "where" on Earth part of your question, most places where
the timing is right will be fine. One difficult part of the planet
to observe this are the poles (makes sense because the extreme poles
have 24 hour "days" and "nights" during winter and summer). At
the poles, you'd have to be very close to spring (the vernal equinox)
or fall (the autumnal equinox) to observe moonrise and sunset.
Thanks for your interesting question! Please feel free to write again
with more quesitons or for clarification!
Regards,
Debra Fischer
Back to the
Ask An Astronomer page.