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Public Observing for the Transit of Mercury, 11 November 2019

Topic: Mercury / Transit
Article Published - October 29, 2019
by Dr. Jim Lattis, Director, UW Space Place

When: Monday, 11 Nov 2019, 6:30am until noon, weather permitting
Where: Washburn Observatory, 1401 Observatory Dr., Madison

The Astronomy Department of the University of Wisconsin-Madison will open Washburn Observatory to the public for safe viewing of this event, weather permitting. Drop by to get a rare glimpse of Mercury on the Sun.

Mercury Transit
Remember the Transits of Venus of 2004 and 2012? In those rare events (there won't be another until December 2117) we saw our neighbor planet in the form of a small, black disk cross, or transit, the bright disk of the Sun. The event is analogous to a solar eclipse, in which the Moon crosses the face of the Sun, although no planet's disk can completely block the disk of the Sun as the Moon can. A Mercury transit is similar to a Venus transit, but Mercury makes a much smaller disk on the face of the Sun because it is both smaller than Venus and farther away. Another difference is that Mercury transits are rather more common than Venus transits, happening 13 or 14 times per century compared to over a century between pairs of Venus transits.

Mercury transits typically occur in pairs, one in May, followed by one in November, about 3 and a half years apart, although the May transit is often skipped. The most recent was May 2016. After this year's event, the next Mercury transit will occur 13 Nov 2032.

Seen against the solar disk, Mercury's disk is so small that a telescope is required to make it visible. In fact, it would take over 150 Mercury disks to span a solar diameter! (This in contrast to Venus, whose disk could be seen on the Sun without magnification.) The diagram shows the path that Mercury's disk will take across the face of the Sun giving times as seen from Madison. Note that the transit is already in progress at sunrise.

Advice: Aiming a telescope or binoculars at the Sun is a dangerous operation, requiring special equipment and techniques, and therefore best left to experienced observers. A full-aperture solar filter properly fitted to the front of the telescope is a good way to observe this event, but make sure no finder telescopes or other devices are exposed to the direct sunlight. It is possible to project the image of the Sun formed by a telescope or binoculars onto a screen, and the image projected on the screen is safe to observe. However there is still a serious hazard in the intense beam of sunlight, which can cause instantaneous eye damage and even start fires. Moreover, many telescopes and binoculars will be damaged by the intense heat, while finder telescopes and sights present further dangers. Do not look directly at the Sun without appropriate filters, and do not point optical equipment at the Sun unless you know exactly what you are doing.

N.B. In case of extensive cloud cover, this event will be canceled. Also note that public parking on campus is very limited on weekdays.


From the University of Wisconsin's "Space Place"
Villager Mall, 2300 S Park Street, Madison



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