Mercury Transit

Join with the Wilderness Center Astronomy Club on the morning of Monday, November 11, to watch Mercury transit or pass directly between the Earth and the Sun for the first time since May 2016 – a sight that will not occur again until November 2032.

Mercury transiting the sun.

We will be watching the transit with people from across the Americas, Canada, Africa, Europe and French Polynesia.  The entire Mercury transit will take 5 hours and 28 minutes. For our location the transit begins at 7:36 am (about 26 minutes after sunrise) with first contact. The midpoint occurs at 10:20 am and the transit ends at 1:04 pm with last contact.

What is transit? Transit across the Sun takes place when an inner planet passes directly between the Sun and the Earth.  As seen from Earth only the planets Mercury and Venus can be seen transiting the Sun and they are relatively rare occurrences.  Mercury transits the Sun 13 or 14 times each century.  Transits of Venus are even rarer occurring only in pairs with more than a century passing between pairs of transits.

Transit of Mercury (indicated by white bars) as seen by NASA Mars Rover Curiosity.

So why are transits interesting?  Well, the transit of Venus was used to figure out the Astronomical Unit, the average distance from the Earth to the Sun, by measuring where Venus appeared to be on the Sun’s disk at the same time from different locations on Earth.  Today, transits are used by astronomers to locate exoplanets around other stars.  They use the transits of Venus and Mercury as exoplanet analogs.  They provide a model for taking measurements and detecting planets around other suns.

On June 3, 2014, NASA’s Mars Rover Curiosity observed a transit of Mercury across the Sun.  This is the first time a planetary transit was observed from a planet other than Earth in our solar system.  Also visible in the Mars – Mercury transit image are two sunspots, both about as big as Earth.

Why are there so few transits of Mercury? It all comes down to planetary alignment and geometry.  Mercury’s orbit is inclined at 7 degrees to the Earth-Sun plane.  The drawing below shows this inclination of Mercury’s orbit with respect to the Earth-Sun plane.  Mercury’s orbit is not to scale, but has been exaggerated to show the geometry. Because of this incline, Mercury is usually either above or below the Earth-Sun plane when it passes in front of the Sun as seen from Earth.

Mercury passes through the Earth-Sun plane about every 44 days as it orbits the Sun. This crossing point is called a node.  When Mercury is between the Earth and Sun, it is called inferior conjunction. A transit can only occur when Mercury is at inferior conjunction when it crosses through the Earth-Sun plane.

Stellarium image of November 11, 2019 Mercury transit.

What will you see through the TWC telescopes?  Mercury’s diameter is 1/194 that of the Sun as seen from Earth.  That is 150 times smaller than the Sun. We are talking really small!  In the telescope or binoculars, Mercury will appear as a tiny black dot against the Sun’s disk. See the Stellarium image of the November 11 transit below.  This is what you will see.  Here a safety warning is needed.  The Sun is a dangerous object to observe.  Never ever, look directly at the Sun with unprotected eyes or observing through a telescope or pair of binoculars without proper filtering.  The result can be instant and permanent blindness.  The advantage of viewing the transit with an astronomy club is they known how to properly observe the Sun and have the correct equipment to do so safely.

If you want to observe the transit of Mercury on your own, please follow these precautions:

  • Don’t ever look at the Sun without proper eye protection. This cannot be said enough times.  Your eyesight depends on it.
  • Don’t view the Sun through sunglasses of any type.
  • Don’t use filters made from photographic film, or combination of photographic filters, crossed polarizers, or gelatin filters, CDs, CD-ROMs or smoked glass.
  • Don’t fit any filter to a telescope or binoculars unless the filter is made specifically for safe solar viewing. Use the filter only after checking it thoroughly for damage.  If it is scuffed, scratched, has pinholes in it, or you have any doubts about it at all DO NOT use it.  Safe filters include aluminized Mylar filters or black polymer filters, identified as suitable for direct viewing of the Sun, bearing the CE mark AND a statement that it conforms to European Community Directive 89/686/EEC.  These cautions also apply to any solar viewing glasses you might want to use.  Always read and follow the manufacturer’s instructions carefully.
  • Don’t leave equipment being used to project an image of the sun onto a safe viewing surface such as white paper on the ground unattended. Don’t let anyone look through the unprotected telescope eyepiece or finder scope or binoculars.

We hope to see you on November 11.  Even if you can’t stay for the entire Mercury transit event you can enjoy watching part of it.  Of course, if it’s cloudy the transit watch is cancelled.


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