How Far Away Is The Sun? A Transit Of Mercury Next Month Will Allow Citizen Scientists To Check

Mercury will ″transit″ the Sun on November 11 in one of 2019's most important celestial events.

Mercury will "transit" the Sun on November 11 in one of 2019's most important celestial events.

NASA

*This article has been updated to include quotes from Zack Stockbridge, co-ordinator of the CitizenToM project*

On Monday, November 11, 2019 something very rare will happen in the daytime sky above the Americas, Western Europe and West Africa. Last occurring in May 2016, but not predicted to happen again until November 2032, the tiny planet Mercury will move across the disc of the sun.

This “transit” of Mercury, as astronomers call it, will take about 5.5 hours and be best viewed from the eastern U.S. where the entire event is visible from breakfast through lunch (you can find out exact times for your location here), though it will also be visible in Europe, Africa, South America and partially in the western U.S.

Cue a data-gathering event called CitizenToM, a U.S.-wide citizen science project to take time-synchronized images of the black dot of Mercury on the sun’s disc from various locations in the country.

The mission is to use the transit—one of astronomy's rarest predictable events—to measure the size of the astronomical unit (au), the average distance from the Earth to the sun. It's the unit that astronomers use to measure the solar system, though its exact value is also known: 149,600,000 km (though since Earth’s orbit is elliptical the actual distance is smaller and sometimes larger than 1 au).

The outer planets are often described as being however many au’s from Earth (though that constantly changes as the planets orbit the sun). For example, Neptune’s average distance from the sun is 30.1 au.

What is a transit?

Planetary transits occur when a planet crosses the face of their star, though they’re only experienced from a certain point of view. From Earth, only the two innermost planets in the solar system, Mercury and Venus, can be seen to transit the disc of the sun. The outer planets never get between Earth and the sun, though it’s possible to see transits from other planets. You can’t see a transit of Earth across the sun unless you’re on Mars or further out.

By the way, the next transit of Earth as seen from Mars will take place on November 10, 2084. Transits are rare!

What is CitizenToM?

An offshoot of CitizenCATE that imaged the “Great American Eclipse” from across the US on August 21, 2017, CitizenToM will have amateur astronomers and school kids in locations relatively far apart within the U.S. all taking observation notes on timings for the ingress (when it begins to cross the sun’s disk) and egress (when it exits the sun’s disk) of Mercury. “The technique is actually very straightforward,” says Zack Stockbridge, co-ordinator of the CitizenToM project and a maths teacher at Southwestern Community College in Sylva, North Carolina. “We're going to have as many observing sites as we can spread as far apart as we can and everyone will use identical equipment. That's important because it keeps the image size for the sun the same and we don't have to deal with resizing things.”

At specific times, participants will take short videos of Mercury’s position on the sun’s disk simultaneously and later extract still images that can be compared. There’s more info on CitizenToM on this Facebook page.

How often do transits of Mercury occur?

About 13 times a century, always in either May (when we're relatively close to Mercury) or November (when Mercury is closer to the sun). That dependable pattern occurs because Mercury's 88 day orbit is more or less resonant with Earth's 365 day orbit (88x4 = 352). That slight difference of 13 days will mean it won't always be May and November, but the opportunity to see a ToM will always be six months apart. The last transit of Mercury occurred on May 9, 2016.

However, it's transits of Venus (which last occurred in 2004 and 2012, and won't happen again until 2117 and 2125) that are much more famous in astronomy than transits of Mercury, even though only seven of them have ever been observed.

How will CitizenToM measure the astronomical unit using the transit of Mercury?

The mathematical basis is the solar parallax; you put observers at different positions on Earth’s surface and take photos of exactly where the planet appears to be against sun’s disk, then use some simple triangulation trigonometry to calculate the distance from the Earth to the sun.

CitizenToM will have around 20 observing sites and everyone will use identical equipment to take simultaneous images of Mercury during the transit. With everything perfectly synchronised, the parallax effect will show up as Mercury's disc shifting positions on the sun. “Based on the distances between the observing sites, we will see the parallax effect and see Mercury's disc shifting positions on the sun,” says Stockbridge. The images will be about two megapixels, though Mercury is only going to shift at most 10 pixels in the image. “Most participating sites have gotten some practice observations in and everything looks good so far,” he adds.

Amateur astronomers will be out in force on November 11. “My students and I will be timing the ingress and egress of Mercury, then reporting our data to others from a long baseline across the country, where it will be compared to the other results,” says Charles Fulco, a NASA Solar System Ambassador who will be observing and hopes to contribute to CitizenToM. “We will be replicating the same mathematics that took place in previous centuries, hoping to establish a fairly accurate value for the astronomical unit.” Fulco will be at his “lucky spot” on the boardwalk at Playland Amusement Park in Rye, NY, on November 11, 2019, where he's observed four transits.

NEVER look at the sun with the naked eye or through telescopes/binoculars unless they have special solar filters.

You can also try to have a look through any eclipse glasses you may still have leftover from the “Great American Eclipse”, though Mercury is going to look pretty small (it may be too small to see).

Closer to November 11 I will post an update on public events where members of the public can get a chance to look through a solar telescope during the transit, which is the best option for most people.

Wishing you clear skies and wide eyes.

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*This article has been updated to include quotes from Zack Stockbridge, co-ordinator of the CitizenToM project*

On Monday, November 11, 2019 something very rare will happen in the daytime sky above the Americas, Western Europe and West Africa. Last occurring in May 2016, but not predicted to happen again until November 2032, the tiny planet Mercury will move across the disc of the sun.

This “transit” of Mercury, as astronomers call it, will take about 5.5 hours and be best viewed from the eastern U.S. where the entire event is visible from breakfast through lunch (you can find out exact times for your location here), though it will also be visible in Europe, Africa, South America and partially in the western U.S.

Cue a data-gathering event called CitizenToM, a U.S.-wide citizen science project to take time-synchronized images of the black dot of Mercury on the sun’s disc from various locations in the country.

The mission is to use the transit—one of astronomy's rarest predictable events—to measure the size of the astronomical unit (au), the average distance from the Earth to the sun. It's the unit that astronomers use to measure the solar system, though its exact value is also known: 149,600,000 km (though since Earth’s orbit is elliptical the actual distance is smaller and sometimes larger than 1 au).

The outer planets are often described as being however many au’s from Earth (though that constantly changes as the planets orbit the sun). For example, Neptune’s average distance from the sun is 30.1 au.

What is a transit?

Planetary transits occur when a planet crosses the face of their star, though they’re only experienced from a certain point of view. From Earth, only the two innermost planets in the solar system, Mercury and Venus, can be seen to transit the disc of the sun. The outer planets never get between Earth and the sun, though it’s possible to see transits from other planets. You can’t see a transit of Earth across the sun unless you’re on Mars or further out.

By the way, the next transit of Earth as seen from Mars will take place on November 10, 2084. Transits are rare!

What is CitizenToM?

An offshoot of CitizenCATE that imaged the “Great American Eclipse” from across the US on August 21, 2017, CitizenToM will have amateur astronomers and school kids in locations relatively far apart within the U.S. all taking observation notes on timings for the ingress (when it begins to cross the sun’s disk) and egress (when it exits the sun’s disk) of Mercury. “The technique is actually very straightforward,” says Zack Stockbridge, co-ordinator of the CitizenToM project and a maths teacher at Southwestern Community College in Sylva, North Carolina. “We're going to have as many observing sites as we can spread as far apart as we can and everyone will use identical equipment. That's important because it keeps the image size for the sun the same and we don't have to deal with resizing things.”

At specific times, participants will take short videos of Mercury’s position on the sun’s disk simultaneously and later extract still images that can be compared. There’s more info on CitizenToM on this Facebook page.

How often do transits of Mercury occur?

About 13 times a century, always in either May (when we're relatively close to Mercury) or November (when Mercury is closer to the sun). That dependable pattern occurs because Mercury's 88 day orbit is more or less resonant with Earth's 365 day orbit (88x4 = 352). That slight difference of 13 days will mean it won't always be May and November, but the opportunity to see a ToM will always be six months apart. The last transit of Mercury occurred on May 9, 2016.

However, it's transits of Venus (which last occurred in 2004 and 2012, and won't happen again until 2117 and 2125) that are much more famous in astronomy than transits of Mercury, even though only seven of them have ever been observed.

How will CitizenToM measure the astronomical unit using the transit of Mercury?

The mathematical basis is the solar parallax; you put observers at different positions on Earth’s surface and take photos of exactly where the planet appears to be against sun’s disk, then use some simple triangulation trigonometry to calculate the distance from the Earth to the sun.

CitizenToM will have around 20 observing sites and everyone will use identical equipment to take simultaneous images of Mercury during the transit. With everything perfectly synchronised, the parallax effect will show up as Mercury's disc shifting positions on the sun. “Based on the distances between the observing sites, we will see the parallax effect and see Mercury's disc shifting positions on the sun,” says Stockbridge. The images will be about two megapixels, though Mercury is only going to shift at most 10 pixels in the image. “Most participating sites have gotten some practice observations in and everything looks good so far,” he adds.

Amateur astronomers will be out in force on November 11. “My students and I will be timing the ingress and egress of Mercury, then reporting our data to others from a long baseline across the country, where it will be compared to the other results,” says Charles Fulco, a NASA Solar System Ambassador who will be observing and hopes to contribute to CitizenToM. “We will be replicating the same mathematics that took place in previous centuries, hoping to establish a fairly accurate value for the astronomical unit.” Fulco will be at his “lucky spot” on the boardwalk at Playland Amusement Park in Rye, NY, on November 11, 2019, where he's observed four transits.

NEVER look at the sun with the naked eye or through telescopes/binoculars unless they have special solar filters.

You can also try to have a look through any eclipse glasses you may still have leftover from the “Great American Eclipse”, though Mercury is going to look pretty small (it may be too small to see).

Closer to November 11 I will post an update on public events where members of the public can get a chance to look through a solar telescope during the transit, which is the best option for most people.

Wishing you clear skies and wide eyes.

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I'm an experienced science, technology and travel journalist interested in space exploration, moon-gazing, exploring the night sky, solar and lunar eclipses, astro-trave...