You may’ve seen some folks writing about this weekend’s so-called Supermoon. I suppose I’m not surprised, but it’s still irritating. Why? Because it’s just hype (and to get this out of the way immediately, will have no real effect on the Earth, either). Here’s the scoop.
This weekend, on the night of May 5/6, the Moon will be full. This happens every 29 days or so when the Moon is opposite the Sun in the sky, and we see its face fully illuminated.
As it happens, the Moon’s orbit is elliptical, and so sometimes the Moon is a bit closer to the Earth than other times. Every now and again the Moon is full when it’s also closest to Earth — the point in its orbit called perigee. May 5th is one of those times.
What does this mean? Well, it means the Moon is closer, so it will appear a bit bigger and brighter than usual. But here’s the thing: you’d never know. Seriously, to the eye it’ll look exactly the same as it always does when it’s full. The Moon is actually pretty small in the sky — if you don’t believe me, go outside tonight, find the Moon, and hold your thumb up at arm’s length next to it; it’ll easily cover the Moon entirely (my thumb is 2 – 3 times wider than the Moon). A small change in its size is something that’s really hard to see.
To be specific, according to Fourmilab, the Moon will be 356,953 kilometers from Earth when it’s full. However, last month, on April 7, when it was full it was about 358,313 km away. That’s a difference of 1400 km, less than 1%. So really, the size of the full Moon this weekend won’t be any different than it was last month, and no one was writing about it then. And to show I’m not being biased, take a look at when the Moon was full near apogee — the most distant point in its orbit. That’ll happen in late November of 2012, when it’ll be at a distance of 406,364 km. That’s still only a difference of less than 14%.
That’s a pretty small change, not enough to notice by eye. Read More
[On January 4, 2012, I started a new features: BAFacts, where I write an astronomy/space fact that is short enough to be tweeted. A lot of them reference older posts, but some of the facts need a little mathematical explanation. When that happens I’ll write a post like this one that does the math so you can see the numbers for yourself. Why? Because MATH!]
From Pluto, the Sun is fainter than it is from Earth, but still can be 450x brighter than the full Moon.
I remember reading a science fiction story many years ago which took place on Pluto. The author described the Sun as being so faint that it looked like just another bright star (too bad I don’t remember the name of the story anymore). I was thinking about that again recently, and wondered just how bright the Sun does look from Pluto. This turns out to be pretty easy to calculate!
First, you need to understand how an object like the Sun — really, any source of light — dims with distance. The Sun emits light in all directions, so as you get farther away from the Sun, that light gets spread out. Imagine a sphere perfectly encasing the Sun right at its surface. Each square centimeter has a certain amount of light passing through it. If I double the size of the sphere, there’s a lot more surface area to that sphere, but the total amount of light passing through it hasn’t changed. Therefore the amount of light passing through each square centimeter has dropped. Since I doubled the sphere’s diameter, I can figure out how much its dropped, too!
The formula for the surface area of a sphere is
Surface area = 4 × π × radius 2
If I double the size of the sphere, everything on the right side of the equation stays the same except for the radius, which is now twice as big. Therefore the area increases by 22 = 4. So the light passing through each square centimeter of the bigger sphere drops by a factor of four. Someone standing on that sphere would see the Sun being 1/4 as bright as if they were on the surface.
If I make the sphere ten times bigger, the area goes up by 10 × 10 = 100 times, and the brightness drops by 100. You get the picture.
So now we’re ready to figure out how bright the Sun is from Pluto!
The Earth orbits the Sun, on average, at a distance of about 150 million km. Pluto has a very elliptical orbit, but has an average distance of about 5.9 billion kilometers, or roughly 39 times the Earth’s distance from the Sun. Using the method above, the Sun must be 392 = about 1500 times fainter, or more grammatically correctly, 0.00065 times as bright. That’s pretty faint!
Or is it? Well, let’s compare that to how bright the full Moon looks from Earth. To us here at home, the Sun is about 400,000 times brighter than the full Moon, so even from distant, frigid Pluto, on average the Sun would look more than 250 times brighter than the full Moon does from Earth!
Pluto’s orbit is also highly elliptical, stretching from 4.4 billion km to just over 7.3 billion km from the Sun. Doing the math again, that means the Sun goes from being 0.0012 to 0.0004 as bright as it is from Earth: a range of roughly 150 to 450 times as bright as the Moon from Earth. That’s a change in brightness by a factor of three!
Still, given that you can read by the light of the full Moon, obviously the Sun from Pluto is still pretty dang intense. It would hardly look like just any other star: it would greatly outshine everything else in the sky. Painful to look at, most likely. So the short story I read was wrong, but at least we learned something. That’s a decent trade.
And let me leave you with a question: From Pluto, how big would the Sun look? Ah, that’s a BAFact for another day. Tomorrow, actually!
I almost missed this, but an email from astrophotographer Anthony Ayiomamitis (whose photo I feature below) reminded me: tonight’s full Moon occurs at apogee, the point in the Moon’s orbit where it is most distant from Earth. I actually wrote quite a bit about this last year, so I’ll repost the article below. Full Moon occurs officially tonight at 02:06 UTC (10:06 p.m. Eastern US time), so in a couple of hours as I write this. Apogee occurs about 9 hours later (October 12 at 11:44 UTC), when the Moon will be 406,176 km (252,286 miles) from the Earth. It was at perigee on September 28, when it was a mere 357,555 km (222,174 miles) from us… but make sure you read the footnote below!
And I’ll note: the difference in size between the Moon at closest and farthest approach isn’t something you’d probably never notice it by eye, especially since you can’t compare the two at the same time. The change is gradual, and the Moon is actually pretty small in the sky. But it’s still neat when you take a picture and compare them…
I’ve been posting a lot of extreme close-ups of the Moon, but sometimes you can learn something by taking a step back.
For example, I imagine if I went out in the street and asked people what shape the Moon’s orbit was, they’d say it was a circle (or, given recent poll results, they’d say it was Muslim). In fact, however, the Moon’s orbit is decidedly elliptical. When it’s closest to Earth — the point called perigee — it’s roughly 360,000 kilometers (223,000 miles) away*, and when it’s at its farthest point — apogee — it’s at a distance of about 405,000 km (251,000 miles).
That’s a difference of about 10% — not enough to tell by eye, but certainly enough to see in a picture… like this one, by the Greek amateur astronomer Anthony Ayiomamitis:
[Click to emperigeenate.]
Amazing, isn’t it? The Moon is noticeably different! He took those images at full Moon, but seven months apart, when the Moon was at perigee (last January) and apogee (just a few days ago as I write this). It’s part of a project he does every year, and it’s pretty cool. He was able to get these images within a few moments of the exact times of apogee and perigee.
You might wonder how the Moon can be at apogee when it’s full one time, and perigee at another time it’s full. Read More