The November 8, 2006 Transit
of Mercury

History | Observing | DAS Press Release
ONLINE Reservation System

On November 8th, given clear daytime skies, many members of the DAS, perhaps numerous members of the public, media, and Dr. Robert Stencel, Observatory Director, will be watching a small, round object move across the face of the sun. The tiny black disk will be the planet Mercury. Some of those observing this astronomical phenomenon will be using the 20-inch Alvan Clark refractor, the 6-inch Grubb telescope ‘finder’ or perhaps looking through the 5-inch Clark ‘finder’ on the side of the great 20-inch. Others may be out on the south lawn with their personal telescopes viewing the event.

Given that planets move in nearly circular orbits about the sun along the path called the ecliptic or plane of their orbits, it should be no surprise that occasionally one of the “inferior” planets, Mercury or Venus, passes in front of the sun. And, given that Chamberlin Observatory and its Clark refractor have made 112 orbits of the sun with the Earth since they were constructed, it is not surprising that other transits of Mercury would have been observed with Chamberlin’s 20-inch telescope.

NASA Astronomer Fred Espenak [1] explains the recurrences of transits of Mercury:

“All transits of Mercury fall within several days of 8 May and 10 November. Since Mercury's orbit is inclined seven degrees to Earth's, it intersects the ecliptic at two points, or nodes, which cross the Sun each year on those dates. If Mercury passes through inferior conjunction at that time, a transit will occur. During November transits, Mercury is near perihelion and exhibits a disk only 10 arcseconds in diameter. By comparison, the planet is near aphelion during May transits and appears 12 arc-seconds across. However, the probability of a May transit is smaller by a factor of almost two. Mercury's slower orbital motion at aphelion makes it less likely to cross the node during the critical period. November transits recur at intervals of 7, 13, or 33 years while May transits recur only over the latter two intervals.”

This year, it is of special note, though, that if we could turn the clock back 112 Earth orbits of the sun, we would see Chamberlin Observatory, the Observatory director, his students, and his wife making timings with the same, but brand new great 20-inch telescope, and the 5-inch Clark finder, while others are in the “Student Observatory” adjacent to Chamberlin observing and making timings with the Grubb telescope. Only a few months after first light for the observatory occurred in July, 1894, a transit of Mercury occurred on November 10. These observations and timings would be the first science to be done using Chamberlin’s 20-inch telescope and would be published by the U.S. Naval Observatory.

It was Edmond Halley that realized that if accurate enough timings could be made of the transits of Mercury or Venus, a distance scale of the solar system could be derived. Timing these transits then became an extremely important goal for early astronomers. By 1894 the scale of the solar system was pretty well determined from earlier transits, although more accurate measurements were needed to refine the numbers. Perhaps this was a ‘learning opportunity’ for DU students to learn how to make observations and reduce them mathematically. It was also an opportunity to make possibly important discoveries about the nature of the planet Mercury itself. The physical nature of Mercury was largely unknown at the time due to its small size and close position to the sun. As Chamberlin’s first director, Dr. Herbert Howe noted in his book, “A Study of the Sky” [2]:

“Of Mercury little is known, for it is coy and keeps close to the sun…It is very difficult to make out any markings on Mercury’s disk…There is great uncertainty about the presence of air or water; certain spectroscopic observations indicate that there may be a thin atmosphere, in which water vapor is present.”

The observations reported to the U.S. Naval Observatory read as an observing log of the event, clearly describing all details of how the timings were made, who made them –giving credit to his students and the problems and poor seeing conditions encountered. Tables of reduced timings are reported for three groups of observers. A brief summary of the first descriptions reads:

“I. The instrument used was a 20-inch equatorial refractor, aperture reduced to 12-inches; polarizing eyepiece with a power of 220 diameters. The timepiece was a sidereal chronometer, Frodsham No. 2593 … Mr. Eugene M. Antram counted loudly the seconds indicated by the chronometer face, and I noted according to his counting. I looked for the planet before first contact, but did not see it until the notch was quite large…A ring of light was looked for around the invisible part of the planet’s disk, between first and second contacts, but none was seen. The planet was seen through light clouds and definition was poor…I waited patiently in the hope that the seeing might improve…Finally, at a critical moment geometrical contact seemed to be well seen and time was noted accordingly. I made a hand signal with my hand to two students, Mr. Ralph Brann and Miss Nan McFarland, when I was sure that the second contact had not yet come…”

“During the transit the planet was looked for occasionally to detect evidences of atmosphere, or possible satellites, or shadings on the disk. The definition varied from very bad to fair. “

“II. Herbert E. Russell, associate professor of mathematics in the University of Denver, made the following observations with the equatorial of the Students’ Observatory: …The instrument used was a 6-inch Grubb equatorial, eyepiece Herschelian, with a power of 99 diameters. The timepiece was a Fauth mean time clock with the chronograph. “III. Mrs. Herbert A. Howe used the 5-inch finder of the 20-inch equatorial, equipped with a Herschelian eyepiece magnifying 120 diameters. Her timepiece was the Fauth sidereal clock of the observatory, connected with a chronograph. In the morning, the chronograph pen worked badly and there was no indubitable record of her observations. “

The observatory was the fulfillment of the dream of Dr. Howe, Chamberlin’s first director and of Humphrey Chamberlin, who wanted to bring the joy of astronomy and a great telescope to the ‘wilds’ of Denver, Colorado during the close of the frontier west. For Dr. Howe, this was the beginning of 32 years of contributions to astronomy and teaching students how to observe and measure the objects in the sky. On Earth 112 years have passed, 464 Mercury years and their great refractor continues to observe the sky and provide students perhaps their first opportunities for learning astronomy with all its joys and frustrations.

References:

[1] http://sunearth.gsfc.nasa.gov/eclipse/OH/transit03.html
[2] A Study of the Sky. Herbert A. Howe, 1896. The Chatauqua-Century Press.
[3] Observations Made At the Chamberlin Observatory of the University of Denver. Report of Prof. Herbert A. Howe. Publications of the U.S. Naval Observatory 9. p. 682.

Transit of Mercury

An Interview with John Westfall, ALPO
Wayne Green,
Denver Astronomical Society

October 2006

Transits of Mercury occur in the months of May and November. This November 8th , we get to watch most of a transit from Denver. We plan to join with Dr. Stencel and observe the passage of the Mercury
from first contact around 12:12PM and running until sunset. Transits have 4 main events called contacts: 1st and 2nd contact occur when the planet first touches the limb and when the trailing edge of the planet leaves the limb. The next main events are 3rd and 4th contact, when the planet makes it to the other side of the Sun. These last two will happen after Mercury sets for us here in Denver. Other events
occur when the planet passes sunspots and other features on the Sun's surface.

An email interview with John Westfall of ALPO gives the following observing tips.

With Mercury's disk less than 10 arc-seconds across, it will be necessary to use at least 100X or so for visual observing -- no problem for your 20-in Clark, but more than the usual Coronado Personal Solar Telescope (PST) eyepiece provides, so you may need to use either a shorter FL eyepiece or a Barlow lens with the PST. Also, if you plan to stack webcam frames, I don't recommend stacking more than a few seconds' worth because of Mercury's rapid motion in relation to the Sun. As you will be timing ingress only, the apparent time of First Contact will be when you first observe a notch on the Sun's limb, caused by Mercury starting to encroach on the photosphere. (Looking through the PST may give you some previous warning of when and where on the limb this will happen.) Apparent Second Contact is when you first see light completely around Mercury's disk (i.e., if the "black drop" effect is present, when the black-drop filament finally breaks).

Note any shading between the limbs of Mercury and the Sun, particularly if the shading varies rapidly. It will also be interesting to compare timings made through telescopes of different sizes and magnifications. Note any apparent illumination on Mercury's disk, or a light or dark ring around Mercury as it crosses the Sun. If we are fortunate enough to see Mercury cross a sunspot, any changes in the planet's appearance should be noted. Also, if possible, compare the darkness of Mercury's disk with with sunspot umbrae and penumbrae.

They [sketches] need not be artistic ("pretty") as long as they accurately record the observer's impressions, particularly of faint or brief phenomena that are difficult to image.

I am planning to publish a report on the event, and I'm sure our readers would like to hear about public interest in the event; how many looked through telescopes and their reaction to the event. He offers some words of encouragement: This will be the last transit of Mercury for 10 years, and the most favorably placed one for Denver until 2061. (Note, however, that some of the 2012 transit of Venus will be visible from Denver.) Some effects that may surprise people are (1) the tiny size of
Mercury compared with the Sun, even though Mercury will be closer; (2) How rapidly Mercury will move in relation to the Sun; and (3) how intensely black Mercury's disk will appear.

Image of the Sun taken from Fred Espanak's site, showing the approximate path of the Transit. The view will be a tad different from that seen here in Denver.

Times taken from Fred Espanak's prediction page.