? asked in Science & MathematicsAstronomy & Space · 10 years ago

# Moon coming close to the earth, what does this mean?

So, the moon's coming closer to the earth, whats that mean?

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• Fred
Lv 7
10 years ago

It means that the Earth is getting closer to the Moon.

This happens every month; it's called lunar perigee. The Moon swings out and back in, each time around its orbit, going from around

364400 km (226400 mi) at perigee

to around

406700 km (252700 mi) at apogee

This is due to what is called its orbital eccentricity of about 5.5% (its orbit isn't centered on the Earth; eccentricity measures how much it's off center, relative to its average distance), and the cycle of distances (perigee, apogee) isn't quite in sync with the phases of the Moon (Full Moon, New Moon), so approximately every 9 years, lunar perigee will happen nearer to Full Moon than in between those times.

The Moon's orbit is a complicated beast. There's the synodic month (Moon phases, 29.5 days), the sidereal month (Moon's place against the stars, 27.3 days), the anomalistic month (perigee to perigee, 27.6 days), the tropical month, the draconic month, all somewhat different. And the Moon's eccentricity varies quite a bit, too. For instance, this perigee is actually 356600 km, 7.3% closer than average, rather than the usual 5.5%. [Average distance is 7.9% farther than now. -- 1/1.079 ~ (1 - .073) ]

As far as what happens when the Moon is at or near perigee:

• The tides are somewhat more pronounced, because, while gravitational force goes as the inverse square of distance, the *tidal* gravitational force goes as the inverse *cube* of distance, so a 7.3% closer Moon makes for 26% (= 1/(1 - .073)^3 - 1) greater tidal force than average. And because the Moon is in full phase, and thus in line with the Sun, the Sun's tidal force, though it is several times weaker than the Moon's, is reinforcing the Moon's effect. This reinforcement happens at full and new Moon.

• The Moon will appear 8% larger than average, as opposed to 7% smaller than average when it's at apogee (about two weeks ago, and again about two weeks from now), but this isn't something you're likely to notice, just by looking at it. But side-by-side photos taken at perigee and apogee, shown to the same (apparent) scale, will be obviously different.

• The Moon will appear 16% brighter than average, but again, this isn't something you're likely to notice, just by looking at it, either. It would be like taking one step closer to a lamp that's 12 yards away.

So overall, this is yet another example of media hype raising expectations among the general public of a phenomenon that *is* somewhat remarkable, but there are surely a lot of disappointed people tonight, going outside to see some spectacular Moon-thing in the sky, when all they will actually see is, "Hey, it's only the Moon!"

• 10 years ago

You obviously don't know apogee from perigee. Orbits are very seldom perfect circles, so there are times when the orbiting satellite (in this case, the Moon) is closest (perigee) to the Earth and other times when it is fartherest away (apogee).

What makes this particular Moon a bit unique is that the "Full" phase of the Moon will coincide with the Moon being at perigee, closest to Earth. No great big deal. Just a nice little tidbit of info. The Moon will be 14% closer and appear 30% brighter. But the casual observer isn't very likely to be able to tell the difference.

Source(s): Louisville Astronomical Society
• 10 years ago

the moon will be closer to the earth.

• 10 years ago

the moon isnt coming closer its moving farther away 3 centimeters each year

• Mac
Lv 5
10 years ago

its a rare occurance that happens about every 20 years. The moon will appear bigger and brighter, apparently its a sight to see

• 10 years ago

Since it's closer, the moon will be bigger and brighter than it has been for over nearly two decades!

• 10 years ago

It means that the gravity is only a tiny bit stronger.

• ?
Lv 7
10 years ago

It doesn't "mean" anything; it's an accident of orbital mechanics.