Mercury, the smallest planet in the solar system, will directly pass between the sun and the Earth on May 9.

Every 88 days, carbon rich Mercury completes its orbit around the sun and goes between the sun and Earth every 116 days. However, the planet's orbit around the sun is tilted that it often passes either below or above the Earth's nearest star. A Mercury transit will only become visible when all three: sun, Mercury, and Earth are lined in three dimensions.

After 3.5 years, the next Mercury Transit will be a November transit, which occurs every seven years, and more common than May transits that only happen every 13 to 33 years.

Mercury Transit Visibility

Mercury's transit will take about 7.5 hours to complete and will be visible in most parts of the globe.

Observers from Western Europe, eastern part of North America, western part of North and West Africa and the majority of South America will be able to witness the entire transit, provided there will be clear skies.

Those from the South and North America, eastern half of the Pacific, rest of Africa and most of Asia can only witness most of the transit, while those from Southeast Asia, Eastern Asia and Australasia will not be able to witness the event.

In North America, the transit will commence in the early morning of May 9, while those in the eastern part will see the beginning of the transit after sunrise. Those in the western areas will see the transit in progress as the sun rises.

Those in the western regions of Europe and Africa will be able to witness the transit noon of May 9, while Thailand, Mongolia and China will observe the event in the late afternoon. Western and Eastern Africa observers will have to wait until mid-afternoon to see Mercury transit.

In the UK, the transit will occur in the afternoon to early evening.

Below are the geocentric phases of the Mercury transit with their position angles:

• Contact I: 11:12:19 UT, 83.2 degrees

• Contact II: 11:15:31 UT, 83.5 degrees

• Greatest Transit: 14:57:26 UT, 153.8 degrees

• Contact III: 18:39:14 UT, 224.1 degrees

• Contact IV: 18:42:26, 224.4 degrees

How To Watch The Mercury Transit

Mercury only has a diameter of 1/158th of the sun, which makes the pinhole telescopes ineffective in observing the transit. Retired astrophysicist Fred Espenak advises observers to use telescopes with a 50x to 100x magnification that is equipped with solar filters to prevent ocular damage.

Mercury will become visible after the first contact and before the fourth contact. However, those with Hydrogen-alpha filtration can view the planet against the prominences before the first contact or the chromosphere fourth contact.

Since second and third contacts occur when Mercury is internally tangent to the sun, its observation requires amplification. After the second contact, a "black drop effect" may also be observed. The planet looks as if it is attached to the limb of the sun by a thin thread.

The rare mercury transit can also be viewed live online via Virtual Telescope Project or via Slooh on May 9, 7 am EST.

Photo: Greg Hewgill | Flickr

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