Bob Doyle, Columnist
Next month we will have two full moons, one on Aug. 1 and the second on Aug. 31. This rare occasion is possible because the lunar phase period is 29.5 days.
So if you have a full moon early in a 31 day month, then you can have another full moon at the end of a month. Double full moons in a month occur about once every three years.
In 1999, we had a double full moon in two months, both January and March. February 1999 didn’t have a full moon.
February is the shortest month (28 or 29 days) and since the lunar phase period is longer than February itself, February can miss a full moon if there is one at the end of January.
Below are some interesting facts about our moon some of which are culled from “How We See the Sky” by Thomas Hockey, University of Chicago Press, 2011, ISBN 13: 978-0-226-3477-2 (paperback).
Our moon takes 27.32 Earth days to orbit the Earth. In the exact same time, the moon rotates. This means that the moon has a near side always facing the Earth and a far side turned away from the Earth.
So each full moon, we see the same lunar craters and grey lunar lava plains (called Mare). Because the moon’s orbit is somewhat elliptical and tilted to the Earth’s orbit, about 59 percent of the moon’s surface can be seen from Earth.
The matchup of our moon’s revolution and rotation is no accident as our satellite experiences strong Earth tidal forces that keep one side of the moon (slightly elongated) always facing Earth.
Most of the large moons of the other planets have the same condition; this includes the four big moons of Jupiter.
Our moon’s phase cycle of 29.5 days is slightly longer than it’s revolution period as the moon’s phases depend on the angle our moon makes with the sun.
Each month, the sun appears to move about 30 degrees along its path (ecliptic). This causes the lunar phase period to be a little more than two days longer than its orbital period.
While the full moon seems larger when it appears near the horizon, the full moon is actually closer to us when it is high in the sky.
Then our part of the Earth is facing the moon and the moon is about 4,000 miles closer to us than when rising or setting. The 4,000 miles is approximately the Earth’s radius, which we must look across at the moon’s rising or setting.
The moon’s apparent larger size when near the horizon is called the “moon illusion,” caused by our mind’s comparing the moon to objects seen along the horizon. This effect also takes place for the sun.
The moon’s phases are caused by our viewing the daylight portion of the moon as it orbits around us. (The night side of the moon blends in with the night sky.)
The new phase occurs when the moon seems to pass either north of or south of the sun. On those occasions when the moon and sun line up, we can have a solar eclipse, which happens at least twice a year.
A few days after the new moon, a sliver of the moon can be seen low in the western dusk. This is called the waxing crescent (waxing means growing).
About a week after new moon, the moon appears half full in the evening sky. This phase is called first quarter as the moon has gone through a quarter of its orbit since new phase.
A few days after first quarter, the moon’s phase is called waxing gibbous (both edges of moon are round).
A little more than a week after first quarter, the moon is full, rising about sunset and hanging in the sky all through the night. At full moon, we can have a lunar eclipse when the full moon moves through the Earth’s shadow.
Following full moon, the moon begins to shift into the morning sky. The phase then is called waning gibbous (waning means shrinking).
In about a week after full, the moon appears half full in the morning sky. this phase is called third quarter as the moon has traveled through three quarters of its orbit since new moon.
The last phase is waning crescent, when the moon is seen as slender sliver low in the eastern dawn. Then the moon disappears in the sun’s glare and reaches New Moon phase again.
SKY SIGHTS THIS WEEK: Venus and Jupiter dominate the eastern dawn with Jupiter near the orange star Aldebaran in Taurus.
The brilliant planet Venus is about six times brighter than Jupiter. The moon is full on Wednesday evening, Aug. 1.
At full moon, you mainly see the grey lava plains which cover two fifths of the moon’s near side. There are few shadows, so the lunar craters are not very noticeable through a telescope.
Bob Doyle invites any readers comments and questions. E-mail him at firstname.lastname@example.org . He is available as a speaker on his column topics.