'Tis the Season!

If you're in the northern hemisphere, then you are currently experiencing late fall/early winter type weather. But why do we even have seasons? And why is it warmer near the equator in winter than it is at the poles? The answer is because Earth is tilted!

Most astronomical pictures project Earth as an "upright" body, with the north celestial pole at the top and the south celestial pole at the bottom. But actually, Earth is tilted 23.44 degrees from the poles being perpendicular to Earth's orbit. Earth's orbit around the sun is almost a perfect circle, so if the Earth was upright, it would be very hot at the equator, very cold at the poles, and every day and night would have exactly the same length. But since the Earth is tilted, we experience seasons. Let's look at the image above. Currently, the Earth is just about at the far left position on the diagram. This is how the Earth appears during winter for the northern hemisphere, and summer for the southern hemisphere. Because the north is tilted away from the sun, it's receiving less direct sunlight and is therefore colder. The south is currently angled towards to sun and receiving more direct sunlight, so it's warmer. As the Earth orbits the sun counterclockwise, we reach northern spring and southern fall, a point where neither hemisphere is angled more towards the sun. This is what gives us our intermediate seasons. On the far right, it's northern summer and southern winter, for the same reasons discussed previously, only now the north receives more direct sunlight. Temperatures at the equator don't fluctuate much at all, because the angle at which it faces the sun doesn't change much. So if you are currently freezing cold in the snow, or basking in the sunlight, you have Earth's tilt to thank or blame!

Earth from the ISS

Many pictures have been published on the web of Earth from outer space. Astronauts often take images of Earth from the International Space Station (ISS). The ISS is a satellite in low Earth orbit that houses up to 15 astronauts at one time. It is used to conduct low gravity experiments, monitor Earth's weather and learn how humans and plant life survive in outer space. Using onboard cameras and other instrumentation, astronauts can image every inch of Earth as they fly by. Below is a video compilation of  many images taken from the ISS from August-October of 2011. Note the beautiful aurora which forms at the top of Earth's atmosphere (defined by what looks like a thin yellow shell around Earth). You can also see city lights across all countries, and flashes of lightening within clouds.

Video credits are listed  here: http://vimeo.com/32001208

Earth | Time Lapse View from Space, Fly Over | NASA, ISS from Michael König on Vimeo.

Earth's Magnetic Field

It's a good thing Earth has a magnetic field! If it didn't, we'd be showered with solar radiation 24/7 and probably wouldn't survive for very long! But what is this magnetic field and how does it work? Astronomers have some idea…

The Earth's magnetic field is thought to be caused by a molten iron core deep inside Earth. With fancy math and physics one can prove that a molten electrically conductive material  that is spinning will produce a magnetic field. This magnetic field is very similar to the one a bar magnet produces, just on a much larger scale. A bar magnet always has a north and south pole. If you sprinkle some iron filings around a bar magnet, you will see that they align themselves in a curved pattern extending from the north to the south pole. The iron filings are tracing the magnetic field lines, or in other words the force, produced by the magnet. One would naturally think that the north pole on Earth should represent the north end of the "bar magnet" inside Earth, and the south pole the south end, but it's actually the exact opposite! Let's stop and think… opposite ends of a magnet attract, while similar ends repel. So if a compass always points North, it must be attracted to the southern magnetic pole! Another fun fact about our magnetic poles is that they do not coincide with the physical north and south poles on Earth, they actually move quite a bit each year! In 2005, the north pole was measured to be at about 83N 114W, but today it is further west by roughly 10 degrees! Astronomers and geologists don't fully understand the production of Earth's magnetic field, and therefore don't fully understand why it is moving, especially at such a quick pace. There's much more work to be done on this subject.