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FAQs

What is dark energy?

How do we think about something we can’t see and don’t experience in our everyday lives, but seems to be pushing our universe apart ever faster? Image credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team, CC BY

How do we think about something we can’t see and don’t experience in our everyday lives, but seems to be pushing our universe apart ever faster? Image credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team, CC BY

Instead of pulling galaxies in our universe together, gravity seems to be driving them apart. How can gravity be repulsive, when our everyday experience shows it’s attractive? That’s why we need dark energy …

Why is Earth Day on April 22?

In the late 1960s, there were love-ins, be-ins ... and, like the first Earth Day, teach-ins.  Here are two organizers of the first Earth Day in 1970.  Image via earthday2013funphotos.com

In the late 1960s, there were love-ins, be-ins … and, like the first Earth Day, teach-ins. Here are two organizers of the first Earth Day in 1970. Image via earthday2013funphotos.com

April 22, 1970 – Arbor Day – was the first Earth Day. Today, a common practice in celebration of Earth Day is still to plant new trees.

What’s the real deal with shark attacks?

Photo credit: C. Fallows

Photo credit: C. Fallows

Sharks are incredibly unlikely to bite you. They’re even less likely to kill you. However, we remain fascinated with their ability – and occasional proclivity – to do just that.

What is earthshine?

The moon and Venus over  North Carolina by Ken Christison.

The moon and Venus over North Carolina by Ken Christison.

That glow over the unlit part of a crescent moon – called earthshine – is light reflected from Earth.

Beautiful photos in this post!

What is the smell of rain?

You can tell when rain is coming just by the smell.  'Rain is Coming' by Taylor Dingess via weather-forecast.com

‘Rain is Coming’ by Taylor Dingess via weather-forecast.com

The word for it is “petrichor.” It’s the name of an oil that’s released from Earth into the air before rain begins to fall.

What happens underground when a meteor hits

When a missile or meteor strikes the earth, the havoc above ground is obvious, but the details of what happens below ground are harder to see. Duke University physicists have developed techniques that enable them to simulate high-speed impacts in artificial soil and sand in the lab, and then watch what happens underground close-up, in super slow motion.

What is a glory?

Photo by Judy Sweeny. It's an airplane glory - shadow of the plane with a halo of light around it - which she took out the window flying from Addis Ababa to Dubai.

EarthSky Facebook friend Judy Sweeny captured this glory from an airplane window in March 2015, while flying from Addis Ababa to Dubai.

Glories are common. They seen all the time by people traveling in airplanes. You need the sun to be directly behind your head. In front, you need an ordinary cloud. As you look toward the cloud, look for the shadow of the airplane. The plane’s shadow may be surrounded by a multi-colored circle of light. That’s the glory.

Why does the sky change color before a tornado?

Green sky, via Houston Newsonline.com

Green sky, via Houston Newsonline.com

There are a couple of theories that suggest what creates the strange greenish-yellow color that can precede a storm.

Why a totally eclipsed moon looks red

The image at left is a photo montage. It combines the famous Blue Marble image taken by the Apollo 17 astronauts with an image of a solar eclipse. The composite somewhat simulates the reason a lunar eclipse looks red – because, during a lunar eclipse seen from Earth, a person on the moon would see a solar eclipse, with sunlight streaming around Earth through our atmosphere. At right, a fully eclipsed moon takes on the reddish color of sunlight filtered and refracted through Earth’s atmosphere. Image at right via Jim Fakatselis. Composite image via AstroBob.

During a lunar eclipse, you’ll see the Earth’s shadow creeping across the moon’s face. The shadow will appear dark, like a bite taken out of the moon’s face, until the shadow completely covers the moon. Then, during the breathtaking moments of totality, the shadow on the moon’s face often often suddenly changes. Instead of dark, it appears red. Why?

Are solar eclipses more common than lunar eclipses?

Image by Joshua Valcarcel via Wikimedia Commons

Image by Joshua Valcarcel via Wikimedia Commons

It’s sometimes said that, on a worldwide scale, solar eclipses outnumber lunar eclipses by about a three to two margin. True?