Just before Halloween last year, NASA launched into orbit the improbably named National Polar-orbiting Operational Environmental Satellite System Preparatory Project, which they thankfully shortened to NPP. In its low 800 km (500 mile) orbit it looks down at the Earth to investigate our environment. It only sees a portion of the Earth at any one time, but if you take observations taken during a single day — say, on January 4, 2012 — and stitch them all together, you get this magnificent shot:

[Click to engaiaenate, or download the Big McLarge Huge 8000 x 8000 pixel version.]

Man, the resolution is so high is like you’re actually there.

Oh wait.

In fact, the biggest version is 8000 pixels across, and the Earth is about 8000 miles wide, so the resolution is about a mile per pixel. We’re not seeing the entire hemisphere here, but the view is roughly 8000 km across (judging from the size of the US compared to the view). The big image is 8000 pixels wide, so the resolution of that mosaic is about 1 km/pixel. The Earth is big.

NPP was recently renamed Suomi NPP in honor of Verner Suomi, a pioneer in using satellites in meteorology. I like that we tend to name satellites and space probes after people whose work made those very missions possible, or for people we honor and respect (my favorite is still Sojourner, the Mars rover named after Sojourner Truth… with the bonus of the name being a pun).

Apropos of nothing, I’ll note the images making up this seamless mosaic were taken around the same time the Earth was at perihelion, when it was closest to the Sun in its orbit. There is nothing particularly important about that fact, but still… when I see pictures like this I think about how amazing our planet is, and how wonderfully well-adapted we are to it. Evolution is a stochastic process, a semi-random series of bumps and false starts that literally made us who were are today. But that doesn’t change the feeling of comfort I get when I see a picture of Earth, floating in space, sitting in the brightest and warmest sunlight of the year.

It’s home, and I’m glad we’re taking such a close look at it.

Tonight at roughly 01:00 GMT (08:00 p.m. Eastern time), the Earth will be at a special place in its orbit: perihelion, the closest point to the Sun. Our orbit around the Sun is not a circle, but actually an ellipse, and in early January every year the Earth’s motion sweeps us closest to our favorite star. We’re only a couple of million kilometers closer than average so it’s a small difference, and not one you’d notice unless you were paying very close attention.

If you want a little more precision, the distance from the center of the Earth to the center of the Sun will be 147,097,206.9 km at that moment. More or less.

Apropos of this, I wrote a guest post about perihelion and what it means for the wonderful BBC blog called 23 Degrees. This is the companion blog for a TV documentary series they’re making (to air later this year) where they traveled the globe to film meteorological and astronomical events that occur during the course of one year. And since they began this journey at perihelion last year, I’m honored to have this anchor position.

So to speak, of course. Anyway, check the Related Posts links below for lots more about past perihelia (they’re listed in reverse chronological order). It’s always fun to write about it, and always fun to learn more about this spinning ball of rock we live on and the giant ball of plasma it orbits.

- At the bottom of Earth’s orbit
- Perihelion!
- Does this perihelion make my Sun look fat? (with cool math!)
- Don’t you just hate perihelion?
- Approaching the Sun

Today is the Fourth of July, Independence Day for us American types.

It also happens to be aphelion^*, the point in Earth’s ever-so-slightly elliptical orbit when it’s farthest from the Sun. Perihelion — closest approach — happens in early January, and aphelion six months later. The dates change a bit from year to year because there aren’t an even number of days in a year (that pesky extra 0.24 in the 365.24 days per year messes things up), and there are other minor factors as well.

Today though, aphelion occurs on or about 15:00 UT (11:00 Eastern US time), when the center of the Earth will be about 152,102,715 km (94,512,245 miles) from the center of the Sun — give or take a few hundred meters. If you’re curious, that’s about 1.67% farther from the Sun than on average. That in turn means the Sun appears about 1.67% smaller in diameter than usual, which isn’t noticeable to your eye — and I don’t recommend trying to find out — but is pretty obvious in photographs using telescopes and heavy filtering, like this one from astrophotographer Anthony Ayiomamitis:

Cool, huh? When we’re farther from the Sun we receive a bit less heat, so perhaps those of you suffering from the midwest heat wave can take consolation that it could be worse by a couple of degrees right now.

Later today, coincidentally, I’ll be at a picnic with lots of solar astronomers. What do I say to them? "Hap-helion Fourth of July"? Or, "Enjoy us being at a(1+e) [where a = 1 AU and e = 0.0167] from the Sun today"?

That seems awkward. The thing is, I’m pretty sure a lot of them would get it…

^* I pronounce it app-HEEL-eeyun, if you care.

[Update: My apologies: due to a cut-and-paste error, I had mistakenly listed the perihelion distance as the average distance of the Earth to the Sun (147 versus 149 million km). To avoid confusion, I simply replaced the error with the correct value. The rest of the post is correct since this wasn't a math error but a typographical one, and I used the right value when doing my calculations below.]

Since last July, the Earth has been falling ever closer to the Sun. Every moment since then, our planet has edged closer to the nearest star in the Universe, approaching it at over 1100 kilometers per hour, 27,500 km/day, 800,000 km every month.

But don’t panic! We do this every year. And that part of it ends today anyway.

The Earth’s orbit around the Sun is not a perfect circle. It’s actually an ellipse, so sometimes we’re closer to the Sun, and sometimes farther away. Various factors change the exact date and time every year — you can get the numbers at the Naval Observatory site — but aphelion (when we’re farthest from the Sun) happens in July, and perihelion (when we’re closest) in January.

And we’re at perihelion now! Today, January 3, 2011, around 19:00 GMT (2:00 p.m. Eastern US time), the Earth reaches perihelion. At that time, we’ll be about 147,099,587 kilometers (91,245,873 miles) from the Sun. To give you an idea of how far that is, a jet traveling at a cruising speed of 800 km/hr would take over 20 years to reach the Sun.

Of course, since today is when we’re closest to the Sun this year, every day for the next six months after we’ll be a bit farther away. That reaches its peak when we’re at aphelion this year on July 4th, when we’ll be 152,096,155 km (94,507,988 miles) from the Sun.

Not that you’d notice without a telescope, but that means the Sun is slightly bigger in the sky today than it is in July. The difference is only about 3%, which would take a telescope to notice. Frequent BA Blog astrophotograph contributor Anthony Ayiomamitis took these images of the Sun at perihelion and aphelion in 2005:

This may seem a bit odd if you’re not used to the physics of orbital motion, but you can think of the Earth as moving around the Sun with two velocities: one sideways as it sweeps around its orbit, the other (much smaller) toward and away from the Sun over the course of a year. The two add together to give us our elliptical orbit. The sideways (what astronomers call tangential) velocity is about 30 kilometers (18 miles) per second, which is incredibly fast. But then, we do travel an orbit that’s nearly a billion kilometers in circumference every year!
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Does the Sun look a little brighter to you? Maybe that’s because at nine minutes after midnight (UT) tonight, January 2/3, the Earth will be at perihelion, the closest point on its elliptical orbit to the Sun.

At that moment, the Sun’s center will be 147,098,040 kilometers away from the Earth’s center (that’s 91,402,484.5 miles for you Murricans). That is, assuming the distance from the centers of the two bodies is 0.983289667 Astronomical Units, and one AU is 149,597,870.7 kilometers. You can compare that to when we reach aphelion, our most distant point from the Sun, which in 2010 will occur on July 6 at 11:30 UT, when we’ll be 1.016701958 AU or 152,096,448 km (94,508,351.3 miles) from our star.

That change in distance — about 5 million kilometers, or 3 million miles — is only a small fraction of our distance from the Sun, so it doesn’t change the Earth’s temperature very much: a few degrees Celsius, but that’s about it. So, of course, that’s not the reason we have seasons. If it were, then we’d have winter in July in the northern hemisphere! But of course, the international cabal of astronomers covers this fact up.

Still, when you think about it, the Sun is a frakkin’ long way off. Even now, at our closest point, it would take over 20 years to fly to the Sun in an airplane at 800 kph (500 mph)! And the TSA would make you sit silently with nothing in your lap for the last 3 years of the journey, too.

But my point is (in case you were wondering if I had one) that the Sun is hot, and there’s a lot of it. I’m glad it’s so far away, even when it’s at its closest.