Tonight, the moon will lie near the horns of Taurus the Bull. And, since it orbits earth in 27.3 days (a “sidereal month”), it will happen again on March 13. In the meantime, it will pass through the 12 zodiac constellations that straddle the ecliptic (earth’s orbital plane). That means the moon moves eastward by 13.1 degrees (360 divided by 27.3) per day.
Earth circles the sun in slightly more than 365 days, advancing slightly less than one degree per day in its orbit. As a result, we see the sun move eastward slightly less than one degree per day relative to the stars, which is why different constellations are visible throughout the year.
With the moon moving 13.1 degrees eastward, and the sun moving almost one degree eastward per day, the moon increases its distance from the sun by 12.1 degrees daily.
Earth rotates one degree in about four minutes. So, all told, the moon should rise about 48 minutes later each day, on average. In reality, the time varies considerably due to various factors, including the latitude and season. An extreme example is the “Harvest Moon” (the full moon nearest the autumnal equinox), when the time of moonrise advances by less than half an hour per day in Idaho.
A curious consequence of the gap between moonrises being a little more than 24 hours is that occasionally a full day will elapse with no moonrise. An example comes later this month: The waning gibbous moon rises at 11:31 p.m. on Saturday, Feb. 23, then not again until 12:39 a.m. on Monday, Feb. 25. Of course, that doesn’t mean the moon isn’t above the horizon for an entire day — only that it’s already (barely) above the horizon at 12:01 a.m. on the 24th.
Be the first to know
Get local news delivered to your inbox!