Enjoying EarthSky? Subscribe.

103,507 subscribers and counting ...


Can you explain why Earth has four seasons?


Happy equinox!

Nearly everyone enjoys the change of seasons on Earth – from winter to spring, from summer to fall. But why do our seasons change?

What do Phobos and Deimos look like from Mars?

Photo Credit: NASA

Photo Credit: NASA

The Martian moons are tiny. The larger moon, Phobos, is only about about 14 miles across. And Deimos is about half that size. They orbit Mars more closely than our moon orbits Earth, but because they’re so tiny they appear smaller than our moon does.

In fact, Deimos, the more distant moon, looks like a star in Mars’ sky. But it’s twice as bright as any star-like object seen in Earth’s sky. Deimos orbits at nearly the same speed Mars rotates – so it needs three Martian days to crawl from one side of Mars’ sky to the other. And, by the way, a day on Mars is about the same length as Earth’s day.

On the other hand, Phobos – the larger and closer of the two moons – zooms around Mars two and a half times every Martian day. Because it out-races Mars’ rotation, Phobos rises in the west and sets in the east. Phobos appears about a third as large in the Martian sky as our moon does in Earth’s sky.

What is a pulsar?

Today’s FAQ: What is a pulsar?

A pulsar is a rapidly spinning neutron star which is the small, incredibly dense remnant of much more massive star. A teaspoon of matter from a neutron star weighs as much as Mount Everest. This cool video from NASA Astrophysics explains it well.

What are the Ides of March, and why beware them?


Today is the Ides of March. Happy Ides!

If you have heard of the Ides of March, you know you’re supposed to beware them. Why? In ancient Rome, the ides of March were equivalent to our March 15th. You probably know of the Ides of March thanks to William Shakespeare. In his play Julius Caesar, a soothsayer – or fortune teller – says to Caesar: Beware the Ides of March.

Stellar luminosity: The true brightness of stars

Image credit: NASA

Image credit: NASA

When we talk about the luminosity of a star, we are referring to the star’s intrinsic brightness. We are not talking about the star’s apparent magnitude – its brightness as it appears from Earth.

For instance, most every star that you see with the unaided eye is larger and more luminous than our sun. The stars that we see at night are millions – even hundreds of millions – of times farther away than the sun. Regardless, you can still see these distant suns because many of them are hundreds or thousands of times more luminous than our local star.

Where is our galaxy in the universe?

Artist's illustration of our Local Group via Chandra X-Ray Observatory.

Artist’s illustration of our Local Group via Chandra X-Ray Observatory.

A word about our Milky Way galaxy within the Local Group, and the Virgo Cluster and Supercluster.

What’s the birthstone for March?

Photo credit:

Photo credit: Wikimedia

Happy birthday to all you March babies! March has two birthstones – aquamarine and bloodstone.

Can the Hubble Space Telescope be used to observe Earth?

Hubble Space Telescope.  Via spacetelescope.org

Hubble Space Telescope. Via spacetelescope.org

If the Hubble Space Telescope (HST) could observe Earth from its orbit 570 kilometers (350 miles) above Earth’s surface, it would in theory be able to see objects as small as 0.3 meters (30 centimeters). But it’s not possible to turn the telescope in an Earth-observing direction. Here’s why.

What would happen if the sun disappeared?

If the sun disappeared, we’d have no idea for eight-and-a-half minutes. And then, cloaked in eternal night, Earth would drift into space.

What’s the birthstone for February?

Hey February babies! Happy birthday! Your birthstone is the amethyst.

The amethyst was the stone of royalty, representing power.

Amethysts contain the second most abundant mineral found in Earth’s crust – quartz. Quartz is often found lining the insides of geodes. So it’s no wonder that geodes sometimes contain amethysts, too. Like quartz, amethysts are a transparent form of silicon dioxide (SiO2). An amethyst’s color can range from a faint mauve to a rich purple. It’s not clear why they’re purple. Some scientists believe the purple color arises from the amethysts’ iron oxide content, while others attribute the color to manganese or hydrocarbons.