This is pretty neat: on June 6, a couple of weeks before the summer solstice, astronauts on the International Space Station pointed a camera to the north and took pictures as they orbited the Earth. Taken over the course of about an hour – 2/3 of a full orbit – this was made into a video where you can see the Sun setting and rising again. What’s cool, though, is the Sun never completely sets. It dips toward the edge of the Earth, then pulls away again:
I love how the Sun shines through the gaps in the solar array.
The geometry of this is fun! Normally, as it orbits the Earth, the ISS passes behind the Earth relative to the Sun, going into the Earth’s shadow. The Earth itself blocks the Sun, so it’s nighttime for the astronauts. Mind you, their orbit is roughly 90 minutes, so this happens on average 18 times per day and lasts for about 45 minutes.
But the ISS orbits the Earth at an angle: the orbit is tilted relative to the Equator by a little over 50°. During the northern hemisphere summer, the Earth’s north pole itself is tilted toward the Sun by about 24°. Combined, this means that for a time around the solstice the ISS can stay in daylight for an entire orbit. The Sun gets very nearly blocked by the Earth, but not quite. I drew a diagram that might help:
The circle represents the Earth. The Sun is off to the left, so the left side of the Earth is lit and the right side is dark. The north pole of the Earth is tipped toward the Sun as shown, and you can see the Equator marked as well. The "terminator" is the day/night line.
I added the rough angle of the ISS orbit – this was done by eye, but shows you how this works. As you can see, the orbit is tilted only a bit from the terminator. Because the ISS is 400 km (240 miles) above the surface, the orbit "pokes over" the edge of the Earth in the diagram (which I exaggerated a bit for clarity). Because of this, the ISS can see the Sun even when it’s over the night side of the Earth: it’s up high enough that the Earth doesn’t block the Sun.
And that’s what the video shows. At the top of its orbit (as shown in the diagram) the Sun gets very close to but not completely blocked by the limb of the Earth’s horizon, and the ISS sees daylight for a full orbit!
Pretty nifty. And look: your tenth grade geometry teacher may have overstated it a bit when she said some day your life may depend on this stuff… but it does make life a lot cooler when you do understand it.
Tip o’ the spacesuit visor to the ESA G+ page.
Today, June 20, at 23:09 UTC (7:09 p.m. Eastern US time), the Earth’s north pole will be tipped over toward the Sun as far as it can for the year. There are other ways to describe it — the Sun reaches its maximum declination, its annual northern movement in the sky peaks, it’s the longest day of the year — but most folks just call it the summer solstice.
You can use this event to measure
your latitude the Earth’s tilt, if you have a stick and a protractor and clear skies and the ability to take an inverse tangent. Or you can read about past summer solstices here, here, here, and here (or the winter solstice here, here, here, here, here, here and here).
Or you can celebrate by checking out the gallery below of some of my favorite pictures of the Sun. If it’s cloudy where you are, or you’re in the southern hemisphere where it’s the winter solstice today, then maybe that’ll help spill a little golden glow into your day.
And finally, think on this: the Earth has had well over 4.5 billion summer solstices since it formed. And it’ll have billions more! Just a little perspective to your day, care of the Universe.
Use the thumbnails and arrows to browse, and click on the images to go through to blog posts with more details and descriptions.
Tonight, at 05:30 GMT (12:30 a.m. Eastern time), it will be the winter solstice. At that moment, the Sun reaches its southernmost declination as astronomers measure position on the sky. What that means to normal people is that every day until June, the Sun’s arcing path across the sky will get a little bit higher, days will get longer, and people in the southern hemisphere will complain that I’m not including them in this description.
OK, fine: if you’re in one of those standing-on-your-head countries, flip the description.
Anyway, the solstice marks the shortest day of the year in the northern hemisphere, more or less (the Earth’s elliptical orbit around the Sun messes with that a bit, changing the length of the day). From here on out, the way the geometry of the Earth’s tilt combines with our orbit around the Sun, our nearest star be up longer every day, getting more time to warm the ground, and eventually heralding the start of summer.
I’ve described this so many times now, it’s probably easier just to send you to the various posts I’ve written about it!
… and the opposite one:
So go enjoy the longest night of the year; take a look at Venus in the west after sunset, Jupiter high in the south all night, Mars rising around midnight, Saturn rising a couple of hours later, and then finally the Moon and Mercury poking over the horizon at twilight. Quite a night for planets!
And if you think about it, look down. There’s another planet there, too.
Or, in other words, it’s the vernal equinox!
A lot of folks will say this is the first day of spring. I think it makes more sense to call this the mid-point of spring — as do many countries — but I’m less inclined to argue about it as much as I used to. What the heck; it’s getting warmer in the northern hemisphere after quite a long and adventurous winter, and I went biking in the sunshine yesterday. It’s sure starting to feel like spring. Good enough for me!
In real terms, the equinox means a few things, too:
So, what are you going to do with your equinox?
Today is the winter solstice — specifically, it occurs at 23:38 GMT, or 6:38 p.m. Eastern (US) time. Technically what this means is that, at that moment, the center of the Sun is at the lowest declination of the year.
Astronomers measure the positions of objects on the sky using a coordinate system based on longitude and latitude on Earth, but to make it more confusing we call it Right Ascension and declination. The first measures the east/west position, and the second north/south. So just like an island might have coordinates of so-and-so degrees longitude and this-and-that degrees latitude, a star has an RA and a dec (if you want to sound cool and use astronomy slang).
OK. But what if an object moves? Well then, the coordinates change with time, too (just like the lat and long of a ship steaming across the ocean has changing coordinates). As the Earth orbits the Sun, the Sun’s position against the background stars changes, and therefore the coordinates change. I won’t go into details — you can read about it here, for example — but the Sun makes a sine wave across the sky over the course of the year, sometimes farther north, sometimes south. The time when the Sun’s position (actually, the center of the Sun’s disk) is at its farthest south — -23° declination — is called the winter solstice.
And that’s where we are today. Read More
Happy winter solstice!
Today, at 17:47 GMT, the Sun reaches the southernmost point in its annual up-and-down journey in the sky. Because the Earth’s axis is tilted, the Sun gets higher in the sky in the summer, and lower in the winter. Today marks the moment when the center of the Sun just kisses that lowest point. From here on out, every day until the summer solstice next June, the Sun will get higher in the sky at local midday.
If you’re in the southern hemisphere, reverse that — replace higher with lower, winter with summer, color with colour, and peanut butter with Vegemite. I’d explain in detail, but that’s what you get for living standing on your heads all the time.
The winter solstice marks the longest night and shortest day of the year. However, it’s not when the Sun rises latest and sets earliest! The Earth has the unfortunate habit of orbiting the Sun in an ellipse, which screws things up a bit when it comes to timing. The actual rise and set times depend on your latitude, but for me, for example, in Boulder, the latest sunrise occurs in the first week of January, and the earliest sunset already happened in early December. Craziness! But the Earth is a crazy place.
If you want more information on this than you could possibly ever need, then try reading the Analemma website. Very cool stuff there. Or you could read what I wrote about this in 2005, 2006, 2007, and 2008.
Still and all, the good news here is that if you live above the equator, the days’re getting longer after today: the Sun will stay up a wee bit more every day. And that means soon it’ll be spring (when eggs will stand on end, just like they do EVERY FRAKKIN’ DAY), and then summer and then autumn and then we’ll be right back here again, as the Earth has done time and again, billions of times, and will continue to do so until the Sun swells into a red giant and consumes it in a fiery blaze of overwhelming solar red gigantism.
But until then, happy new year, too!
Image credit: NASA.