Tag: rotation

Wait just a (leap) second

By Phil Plait | January 23, 2012 7:00 am

Clock at midnightThis summer will be a little bit longer than usual. A tiny little bit: one second, to be precise. The world’s official time keepers are adding a single second to the clocks at the end of June. This "leap second" is needed to keep various time scales in synch. It’s a bit of a pain and won’t really affect people much, but if it weren’t done things would get messy eventually.

This gets a bit detailed — which is where the fun is! — but in short it goes like this. We have two systems to measure time: our everyday one which is based on the rotation of the Earth, and a fancy-schmancy scientific and precise one based on vibrations of atoms. The two systems aren’t quite in synch, though, since the Earth counts a day as a tiny bit longer than the atomic clocks say it is. So every now and again, to get them back together, we add a leap second on to the atomic clocks. That holds them back for one second, and then things are lined up once again.

There. Nice and simple. But that’s spackling over all the really cool details! If you want a little more info, you can read the US Naval Observatory’s press release on this (PDF).

If you want the gory details, then sit back, and let me borrow a second of your time.

Time after time

There are lots of ways of keeping time. The basic unit day is based on the physical rotation of the Earth, and year is how long it takes to go around the Sun. But we need finer units than those! So we decided long ago to divide the day into 24 hours, and those into 60 minutes each, and those into 60 seconds each. In that case, there are 86,400 seconds in a day. OK, easy enough.

For most of us, that is enough. But scientists are picky (or "anal" if you want to be technical) and like to be more precise than that. And the thing is, the Earth is a bit of a sloppy time keeper. Tidal effects from the Sun and Moon, for example, slow it a bit. Other effects come in as well, changing the rate of the Earth’s rotation.

To account for this, in 1956 the International Committee for Weights and Measures made a decision: we’ll base the length of the second on the year, not the day. In fact, we’ll take the year as it was in the year 1900 (a nice round number, so why not) and say that the length of the second is exactly 1/31,556,925.9747 of the year as measured at the beginning of January 1900*.

OK, fine. Now scientists have their anal precise definition, normal people have calendars, and we’re all happy, right?


Sunrise, sunset

Yeah. Not so much. Read More

CATEGORIZED UNDER: Cool stuff, Science, Time Sink, Top Post

Incredible high-resolution video of Jupiter

By Phil Plait | November 18, 2011 7:00 am

The Pic du Midi observatory in France is renowned for its very stable atmospheric conditions, allowing high resolution pictures to be taken. Our air commonly blurs out finer details of astronomical objects; there are ways to compensate, but it’s nice to not have to worry about it in the first place.

So pictures of the planets taken from the 2800-meter-elevation observatory are surpassingly beautiful. I was searching online for some Jupiter info yesterday, and stumbled on a video of the King of the Planets made using observations from Pic du Midi from October 10 – 15, 2001, and, well, it’s stunning. See for yourself:

WOW. Make sure you set the resolution to 720.

I love how it feels like you’re floating over Jupiter as it spins beneath you! Of course, the Great Red Spot is visible, as well as many other circular and highly-elliptical storms. Jupiter is huge, 140,000 km (86,000 miles) across — 11 times the diameter of the Earth. So even in this high-res video, the smallest features you’re seeing are hundreds of kilometers wide!

Despite its enormous size, Jupiter’s day is only about ten hours long. In this video, the bulk motion you see is the planet rotating on its axis, but it’s essentially impossible to see any movement in the clouds themselves. Incredibly, those storms are swept along for hundreds of thousands of kilometers as the planet spins, but in that short time the structure of the clouds hardly changes at all. It’s a study in contrasting velocity.

Right now, Jupiter rises in the east at sunset, making it available all night for observing. When I was in Texas earlier this week the UTPA astronomy folks had some telescopes set up, and Jupiter was a favorite target. All four Galilean moons were visible, and the planet itself showed beautiful detail. If you get a chance to see it through a telescope over the next couple of months, take it! You won’t regret it.

Credit : S2P/IMCCE/OPM/JL Dauvergne/Elie Rousset/Eric Meza/Philippe Tosi/François Colas/Jean Pajus/Xavi Nogués/Emil Kraaikamp

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CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Uranus got double-tapped?

By Phil Plait | October 7, 2011 9:37 am

One of the enduring mysteries of our solar system is why Uranus is tilted over on its side. If you measure the angle of a planet’s rotation axis (the location of its north pole) compared to the plane of its orbit, you find that all the planets in the solar system are tipped. Jupiter is only 3°, but Earth is at a healthy 23° angle; Mars is too. Venus is tipped so far over it’s essentially upside-down (we know this because it spins the wrong way).

Uranus, weirdly, is at 98°, like it’s rolling around the outer solar system on its side. The best guess is that it got hit hard by something planet-sized long ago, knocking it over (though there are other, more speculative, ideas). The problem with that is that its moons and rings all orbit around its equator, meaning their orbital planes are tipped as well. It’s hard to see how that might have happened, even if you assume the moons formed in that collision (as, apparently, our Moon formed in an ancient grazing impact with Earth by a Mars-sized body).

Well, a team of astronomers have come up with a new idea: maybe Uranus wasn’t hit by one big object. Maybe it was hit by two smaller ones.

Read More


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