Tonight – November 5, 2016 – look for the waxing crescent moon to shine near the red planet Mars on the great dome of sky. Carefully note the moon’s position relative to Mars across these two evenings. You’ll see that the moon has moved eastward, away from the sunset direction. The moon moves an average 13o eastward in front of the backdrop stars daily.
However, if you watch the the moon and Mars during the evening hours tonight, you’ll see the moon and Mars shifting westward, toward the sunset direction. These two worlds will follow the sun beneath the horizon by late evening. The moon and Mars travel westward across the sky for the same reason that the sun travels westward during the day: Earth’s rotation.
The Earth rotates from west-to-east on its rotational axis, causing the sun, moon, planets and stars to go westward across the sky each day. Yet, the apparent daily motions of the sun, moon and planets are really a reflection of our planet Earth rotating full circle on its axis in one day’s time.
Actually, we should specify that the Earth rotates full circle relative to the backdrop stars in one stellar day (also called sidereal day). The Earth rotates full circle relative to the sun in one solar day and the moon in one lunar day.
We give the mean period for the stellar, solar and lunar days:
One stellar day (one rotation of Earth relative to the stars): 23 hours 56 minutes
One solar day (one rotation of Earth relative to the sun): 24 hours
One lunar day (one rotation of Earth relative to the moon): 24 hours 50 minutes
We wish to clarify our use of the terms stellar day and lunar day:
We say stellar day, rather than sidereal day, because we are talking about one rotation of Earth relative to the backdrop stars. Sometimes, sidereal day is defined as one rotation of Earth relative to the equinox point, which is a very slightly shorter period of time than one rotation in front of the stars. For most purposes, however, stellar day and sidereal day can be regarded as synonymous.
It is also hard to avoid ambiguity (two meanings) when using the term lunar day. Lunar day is also defined as the moon spinning once upon its rotational axis relative to the sun.
No other celestial body in Earth’s sky moves so swiftly through the constellations of the zodiac as does our moon. As evidence of this, watch how quickly the moon sweeps past Mars in the next few days.
Bottom line: On November 5 and 6, 2016, the moon is near Mars. Note the moon’s position relative to Mars across these two evenings. Its eastward motion on our sky’s dome is due to its eastward motion in orbit around Earth.