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Ephemeris Extra – Transit of Mercury

May 7, 2016

Transit of Mercury – May 9, 2016*

Monday, May 9th the planet Mercury will cross in front the Sun in an event called a transit. Transits of Mercury are not as rare as those of Venus. No one alive who saw that last transit of Venus, will see the next in 2117. The last transit of Mercury was in 2006, and the next will be in 2019, though it’s a long jump to the transit after that in 2032.

Occult4’s geocentric ingress time is 7:12 a.m. (11:12 UT) at position angle 83.1° Farthest penetration onto the Sun’s face is 10:57 a.m. (14:57 UT) Egress time is 2:42 p.m. (18:42 UT) at position angle 224.4°. Position angle is measured from the North point on the Sun counterclockwise.

rack of the Transit of Mercury

The track of Mercury across the face of the Sun. Mercury will travel from upper left to lower right. Mercury will not be visible until it impinges upon the disk of the Sun. Credit IOTA’s program Occult4.

Looking at the Sun normally from northern Michigan, Mercury’s ingress point is close to the 8 o’clock point on its edge, since the Sun will rise tilted to the left nearly 45°. Mercury is tiny, 6.8 seconds of arc in diameter, and will be very hard to spot, smaller than most sunspots. Venus was nearly 58 seconds of arc in diameter when it transited the Sun in 2012.

Since Mercury is invisible before the transit starts. Checking out the Sun in the telescope and moving it in right ascension and declination or altitude and azimuth and altitude so the ingress point can be determined. Newtonian telescopes give an upside down image, actually rotated 180°. Refractors and Schmidt or Maksutov telescopes generally give a mirror reversed image due to the diagonal mirror that the eyepiece is placed into. The image is right side up or upside down depending on the rotation of the diagonal.

The use of a Hydrogen Alpha solar telescope allows an early peek at the transit. These telescopes look at the Sun’s chromosphere, a layer of gas 6,000 miles thick directly above the photosphere. Since the chromosphere is twice as thick as the diameter of Mercury. This should give you a few minutes heads up before white light telescopes can spot the start of the transit. I noticed the effect with the transit of Venus in 2012.

Transit Map 2

Parts of the Earth facing the Sun at the start of the transit (Left) and the end of the transit (Right). At the start of the transit Michigan is near the limb of the Earth at the upper left. The transit starts about 51 minutes after sunrise in norther Michigan. From Occult 4.

The only way to view the transit in white light is with a telescope with a front mounted solar filter. Using an eyepiece to project an unfiltered telescope image with an eyepiece may work, but Mercury is very small and projecting the Sun’s image in the ambient light doesn’t give a contrasty image.

To help everyone out the Grand Traverse Astronomical Society will have telescopes in two locations: The NMC Rogers Observatory and the Dune Climb at the Sleeping Bear Dunes National Lakeshore. Since the transit lasts 6 ½ hours the usual cancellation rules won’t be in effect. If it’s cloudy at the start, it could clear up later on. I’ll be stationed at the Dunes and will be there for the duration, so if we have an all day rain I’ll still be out there, hoping it’ll clear up. Check bobmoler.wordpress.com for the latest on viewing conditions there.

* Based on my article in the May 2016 edition of the Stellar Sentinel, the monthly publication of the Grand Traverse Astronomical Society.

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