Mercury is making a rare transit across the sun on Monday. Here's how to watch.

Stargazers, take note! On Monday, May 9, the planet Mercury will wander directly in front of the sun — a rare "transit" that only happens about 13 times a century.

Mercury begins its transit at 7:12 am EST and will take about 7.5 hours to fully cross the sun's face, finishing at 2:42 pm. The graphic from NASA above shows its expected path.

Most people in Europe and North America should be able to view the event directly through a telescope or binoculars, weather permitting. Just make sure you use a solar filter before watching the transit. If you don't have a filter, don't try to look directly at the sun, not even through your phone camera. You'll risk severe eye damage.

Alternatively, there are quite a few live streams of the transit, like the neat one below from Slooh, a group that collects views from different telescopes worldwide:

Here's a map of where on Earth the transit should be visible, either entirely or in part. In London, for instance, the transit starts at 12:12 pm. Those living on the West Coast of the United States should be able to witness the transit already in progress once the sun rises:

(Europlanet)

You can visit the Night Sky Network to find public viewing events. Again, if you don't have a telescope with a solar filter, there are plenty of internet live streams from Slooh and the European Space Agency and NASA's various satellites.

Here's a fun timelapse of Mercury's journey from NASA's Solar Dynamics Observatory, posted at around 2:30 pm EST:

Mercury leaves the sun's disk in about 20 mins during today's #MercuryTransit. Here's the transit so far:https://t.co/JPHdKtqE3A

— NASA (@NASA) May 9, 2016

Why transits of Mercury are so rare

On average, transits of Mercury occur about 13 or 14 times every 100 years. The last one took place in 2006, and the next one won't occur until November 11, 2019. After that, we won't see another until 2032.

Why so uncommon? For a transit to occur, the sun, Mercury, and Earth all have to line up directly. But Mercury's orbit is inclined by about 7 degrees compared with Earth's. So there are only two spots where the two planets could conceivably line up with the sun — the places where Mercury crosses the Earth's orbital plane:

(European Southern Observatory)

Earth lines up with these intersection spots around May 8 and November 10 each year, give or take a few days. If Mercury, which takes 88 days to orbit the sun, is also wandering through at the same time, a transit occurs. This happens once every seven or eight years.

By the way, a transit of Venus is even rarer. Venus's orbit is inclined 3 degrees from Earth's, but Venus takes 225 days to orbit the sun. That means alignment with Earth happens much less frequently — we've only seen eight of them since the telescope was invented in the 1600s.

Transits of Venus tend to come in eight-year pairs that are then each separated by a century or so. The last two happened in 2004 and 2012, and the next two won't occur until 2117 and 2125. Barring some major medical breakthroughs, most of us won't see one again.

Why the transit of Mercury is so fascinating for scientists

(NASA)

Time lapse of Mercury's transit across the sun in May 2006.

When Mercury crosses the face of the sun, it will be more than just a gawk-worthy spectacle. Scientists will be watching the transit closely to glean all sorts of new insights about space. And this is nothing new: Transits have been teaching us an enormous amount for centuries now.

Back in 1677, astronomer Edmond Halley (of Halley's comet fame) watched Mercury cross the sun and noted that if a transit were to be observed from different latitudes on Earth, the different observers would all see Mercury cross the sun along at a slightly different angle. Those various angles could then be used to calculate the distance between the Earth and the sun — which was still something of a mystery.

Decades after Halley died, during the 1761 and 1769 transits of Venus, scientists from around the world collaborated and used his method to calculate that the distance between Earth and the sun was about 24,000 times Earth's radius. That was only about 3 percent off the real value. Not too shabby.

Then, in the 20th century, as Lyle Tavernier of NASA's Jet Propulsion Laboratory explains, scientists realized they could use transits to study the thin layer of gases that surround Mercury, known as an "exosphere." "Sodium in the exosphere absorbs and re-emits a yellow-orange color from sunlight, and by measuring that absorption, we can learn about the density of gas there," NASA scientist Rosemary Killen tells Tavernier.

During the May 9, 2016, transit, scientists at the Big Bear Solar Observatory in California will try to catch a glimpse of sodium in the planet's exosphere, in order to better understand how it escapes the planet's surface.

Also exciting: When Mercury passes in front of the sun, it causes a slight dip in the sun's brightness. In recent decades, astronomers have realized that they can look for similar brightness dips in distant stars to detect the presence of exoplanets — other worlds that might even contain life:

(NASA JPL)

NASA’s Kepler mission has found more than 1,000 exoplanets using this method. And scientists will be studying the dips in light during Mercury's transit on May 9 to help better refine this method.

Meanwhile, NASA will be using the transit to calibrate instruments on two of its space telescopes, SDO and SOHO — including how to handle stray light on the images they collect.

All in all, this will be the most studied transit of Mercury in history. "It used to be hard to observe transits," SOHO project scientist Joseph Gurman said in a press release. "If you were in a place that had bad weather, for example, you missed your chance and had to wait for the next one. These instruments help us make our observations, despite any earthly obstacles."

Further reading:

• Sky & Telescope has a handy guide for how to look at the sun. Again, do not look at it directly without a filter. Can't say this enough.
• On Monday, NASA is streaming a live program on NASA TV discussing the science of the transit from 10:30 to 11:30 am EDT.
• Mercury was a total mystery for centuries. We're finally learning its secrets.

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