Venus appeared in front of the Sun as seen from the surface of the Earth on June 5th. Called a transit of Venus, it only happens a few times in 250 years. The next one will be in 2117 AD.

Transits of Venus was considered scientific gold for early astronomers, who used them to derive accurate measurement of the size of the solar system. The time each planet took to go around the sun, to crunching that data via methods developed by 17th century mathematician Johannes Kepler, the telescopes could determine each planet's relative distance from the sun.

A Dutch astronomer named Steven van Roode has created an smartphone app, VenusTransit, that lets users simultaneously observe the transit on different sides of the world.  The app notes the observer's precise location on Earth. Then, it collects the data necessary to make the most accurate ground-based measurement of the size of the solar system, while the transit is still happening!

Why doesn't this happen every other year?

The Sun is in the middle of the solar system. We're used to thinking of the solar system as a number line, with all the planets lined up from left to right along the X axis. But that's not the case.

Venus and the Earth go around the Sun at different speeds, in nearly circular orbits—a year on Venus is over before 2/3 of an Earth year is complete. And at any given moment, Venus could be behind the sun, or to the side of the Sun or on the same side of the Sun as seen from Earth. The Sun, Venus, and Earth line up like a string every 584 days. So why doesn't the transit of Venus happen every 584 days?

Th orbit of Venus is tilted 3.25 degrees compared to the Earth's. Which means, that when the Sun, Venus and Earth line up, Venus could be 3.25 degrees above the Sun or 3.25 degrees below the Sun or anywhere in between.

The Sun is about 1/2 degree wide in our sky, and Venus could be in a region of 15 solar diameters wide which is why its not hard to see why this rarely happens.

Image: Dark Sky Photos

When to watch it and where

Venus is now en route, and getting really close. It will last almost seven hours. These are local times to watch the transists:

Honolulu: 12:10pm
Los Angeles: 3:06pm
Mexico DF: 5:06pm
New York: 6:03pm
London: Not visible until sunrise at 4:45 am
Beijing: 6:10am
Hong Kong: 6:12am
Cairo: Not visible until sunrise at 4:52 am
Tokyo: 7:10 am
Sydney: 8:16am

How to watch with your own eyes

• DO NOT LOOK DIRECTLY INTO THE SUN WITHOUT PROTECTION. Staring at the Sun can damage your eyes permanently.

• DO NOT use your sunglasses as protection. They will not protect your eyes while staring directly at the Sun.

• The best protection is wearing welding goggles with No. 14 glass.

• The next best is buying an eclipse filter. They are inexpensive

• Use a telescope or binoculars. Again, these MUST have the proper filter. Most brands come with eclipse filters, so you will be safe.

• Use a pinhole projector. This will allow you to look at the Sun projected on a piece of paper, so it's perfectly safe. You will see the Venus shadow travelling on the paper.

But if you're afraid anything would happen to your eyes you can watch it online

NASA's Transit of Venus, transmitting live from Mauna Kea, Hawaii.
Astronomers without borders.
SkyCenter at Mt. Lemmon.
The exploratorium.
Slooh SpaceCamera (they are live right now).
• The Mead Observatory will have the transit captured live in several wavelengths.
NASA's Astronomy Picture of the Day is also updating live.