Moon watch

The moon is in a waxing (growing larger) gibbous (more than half lit) phase. It will be full on Saturday.

The moon goes completely around the Earth every 27.3 days - which means that it returns to the same relative position in the sky with respect to the background stars every 27.3 days.

However, it takes 29.53 days for the moon to go from new moon to new moon, a lunar month.

The difference in the two periods reflects the Earth's motion around the sun.

The moon does not complete a full lunar cycle until the Earth reaches a point in its orbit where the sun and the moon are in the same relative positions. Since the moon travels completely around the sky every "month," it is constantly changing its location.

If you have the time, this Saturday look at the full moon in the east around 8 p.m. with low-power binoculars to note its relative position in the sky.

Then observe it again Sunday in the pre-dawn sky around 5 a.m.

If you are careful in your observations, you will see the moon has changed its position in the nine hours between your observations. The moon's motion is even more recognizable if you follow it from day to day.

Monday night the 16-day-old moon will be close to Regulus, the bright star in Leo the Lion. Look east around 10 p.m. to see the moon and Regulus about 30 degrees above the horizon. Regulus will be to the left (north) of the moon.

The front part of Leo looks like a large backward question mark. Regulus, the 21st-brightest star in our sky, appears to be the dot on the bottom of that backward question mark. But Regulus is no mere dot - it has luminosity 360 times the sun and is at a distance of 79 light-years.

Contact Tim Hunter at