July 29
The Moon at Apogee
The Moon is at apogee today. For those of you not familiar with astrolingo, that means that the Moon is as far as it is going to get from Earth today, and will start moving closer. The Moon's orbit is elliptical. The actual astronomical term is eccentric. It takes the Moon about 27.3 days to orbit Earth. However, the phases of the Moon depend upon the relative orientation of the Moon, Earth, and the Sun. So, while the Moon has gone all the way around the Earth in 27.3 days, the Earth has moved, so the Moon must go two more days to line up with the Sun again. The phases, thus repeat every 29.5 days. In ancient times, people would mark time by how many times the Moon went through a cycle of phases. So, if something happened "many moons ago" that meant many lunar cycles ago. If you were conducting business and were about to leave on a long journey, and you told your business partner that you would be back in 7 moons, then that meant that you expected to return in a bit over 200 days. The term month came from this cycle. A month is a moonth. Some calendars, such as the Jewish calendar or the Islamic calendar still reckon time in lunar cycles. The months start with the first sighting of the Moon after New Moon. The modern Jewish calendar calculates when this should be. The Islamic calendar still requires someone to actually see the Moon.
But, as the Moon orbits Earth, its orbit is slightly eccentric. Sometimes it is closer, and sometimes farther form us. The difference between closest and farthest from Earth is almost 10%, hardly enough to notice unless you were really carefully measuring how big the Moon appears in the sky. In fact, this difference in size would be pretty close to the naked eye limit for measuring, so you'd need some decent equipment to measure the change in size. The "huge" moon that some people sometimes report seeing is merely an optical illusion due to the Moon being low in the sky near the horizon. I might blog more on this later. Today, the Moon reaches a distance of 405,406km from Earth. It was last at apogee on July 1, when it was 404,448km from Earth, and it will next be at apogee o, August 26, when it will be at a distance of 406,269km (all distances measured from the center of the Earth). But, wait, these distances are all different by a little bit! What gives? Does this happen at the Moon's closest approach, too? Let's see. The Moon was last closest to Earth, what astronomers call perigee, on July 13, when it was 364,288km away. Its next perigee will be August 10, when it will be a distance of 359,749km. So, it seems that the perigee distance must do the same thing. But, why?
Well, if the Earth and the Moon were the only things in the universe, then the Moon's orbit would be nice and perfectly repeating. However, there are other factors involved. The Sun tugs on the Moon and the Earth as they orbit, and that can distort the orbit slightly. This effect will be most pronounced when you look at perigee and apogee near Full Moon or New Moon, when the Earth, Moon, and Sun are all lined up (something called syzygy), and then you compare that to what happens then perigee or apogee occur near First Quarter or Last Quarter Moon, when the line between the Sun and the Earth is perpendicular to the line between the Moon and the Earth. Since the Moon's orbital period (called the sidereal month) is not the same as the lenght of the cycle of phases (called the synodic month), then apogee and perigee slowly shift through the phases. This causes the apogee and perigee distances to slowly change. However, perigee is always the closest that the orbit comes to Earth in that particular orbital cycle, and apogee is always the farthest that the Moon gets from Earth in that particular orbital cycle. Besides solar effects, the Moon's orbit is also affected by the gravitational interactions of the other planets, irregularities caused by massive impacts, and a host of other things. But, the solar effects are by far the biggest effects.
The Moon is just a few days after New Moon, so it will be a crescent Moon in the western sky just after sunset, setting a few hours after sunset. It will be perigee just the day after Full Moon. The tides are highest at Full Moon and New Moon, because tides come from both the Sun and the Moon and they are working together then. The Moon will be particularly close to Earth this next Full Moon, increasing its effects. So, that means that there will be rather higher high tides and lower low tides than normal during the week of the Full Moon in August. Those planning a trip to the beach might want to observe the bigger than normal range of tides then.
Happy Moon watching!
-Astroprof
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