Rare Transit of Mercury

As depicted with Stellarium, the tiny disk of Mercury can be seen set against the brilliant disk of the sun at 8:41 EDT, 9 May, 2016. The orbit of Venus can be seen more closely aligned with the sun’s equatorial region.

Areas where the transit will be visible. For observers in the central Atlantic region, the transit will be observable in its entirety. For observers in the western half of North America, southern Argentina and Chile, the transit will be in progress at sunrise, for most of Africa and sections of Europe east of France and Spain, the transit is in progress as the sun sets.

For the first time since 2006, a rare transit of the planet Mercury will occur tomorrow, May 9 for most of the earth’s surface. Observers in the Eastern Time Zone (Continental US East Coast, SE Atlantic and most of South America) are favorably located to witness the transit in its entirety. With a duration of 7 hours, 30 minutes, the transit begins at 7:12 AM, EDT (11:12:19 UTC) and ends at 18:42 UTC (2:32 PM, EDT). The next such transit of our solar system’s closest planet to the sun won’t occur until 11 November, 2019. There are approximately 13 transits of Mercury each century and although considered rare, by comparison, transits of Venus occur in pairs with more than a century separating each pair!

Geocentric Phases of the 2016 Transit of Mercury
Event Universal Time Position Angle
Contact I 11:12:19 83.2°
Contact II 11:15:31 83.5°
Greatest Transit 14:57:26 153.8°
Contact III 18:39:14 224.1°
Contact IV 18:42:26 224.4°

A brief description of the terms used in the table

Contact I: Mercury’s western limb first “touches” the sun’s eastern limb (Mercury ingress, transit begins)
Contact II: Mercury’s eastern limb touches the sun’s eastern limb
Greatest Transit:  Mercury is closest to the sun’s geometric center
Contact III:  Mercury’s western limb touches the sun’s western limb
Contact IV:  Mercury’s eastern limb touches the sun’s western limb (total egress, the transit has ended)

Although it is possible to observe the transit yourself, two significant challenges need to be overcome: 1) Since Mercury will transiting directly across the brilliant disk of the sun,  a solar filter designed to reduce the sun’s brilliance needs to be used; 2) since Mercury’s angular size is a tiny 12 arc-seconds (a tennis ball observed from one kilometer) as seen from the earth, a telescope with a magnification of at least 50x has to be used.

If you have access to a telescope but not the filter(s), the projection method is a viable option; diligent supervision must  to be exercised however, lest someone inadvertently peer through the unfiltered telescope while aimed at the sun, resulting  in serious eye injury or blindness. If you don’t have access to a telescope or the necessary equipment to observe the transit directly, you can “observe” it via various public observing portals such as Slooh or The Virtual Telescope Project, a public access observing platform hosted and sponsored by the European Union. NASA will provide coverage on their public Television portal, NASA-TV and on their Facebook page from 10:30 to 11:30 a.m., EDT as well as post images acquired from the Solar and Heliospheric Observatory (SOHO) and the Solar Dynamics Observatory to their Transit page.

It is interesting to point out that as we observe Mercury transit across the sun, the Kepler Space Telescope, currently functioning in its K2 Mission, looks for planets orbiting other stars (“exoplanets”) using the “Transit” method, it is as though we are observing Mercury transit the sun just as Kepler observes other planets as they transit their host stars. Since Kepler’s prime directive is “Detection of Earth-like planets in orbit around sun-like stars in the ‘habitable zone'” (a region around every star where water can exist in a liquid state) it must be capable of detecting changes in irradiance of one part in ten thousand or the relative areas of the earth (radius of 6,370 Km) as a disk compared to the sun. Since tiny Mercury’s radius is 0nly 2,440 Km compared to the sun’s 696,000 Km, the change in solar irradiance as Mercury transits the sun will be only one part in 90,000 or 9x lower than the threshold necessary to detect an Earth-like planet transiting a sun-like star.

Positional_astronomyTransits of Mercury and Venus can only occur during “Inferior Conjuction”, a relative position where the two planets are closest to each other and on the same relative side of the sun. Clearly, this is only possible for planets closer to the sun than the observer. If we observed from Mars, we could observe transits of Earth!

Two specific avenues of scientific inquiry will be explored during the transit:

  1. Origin of recently detected traces of Sodium in Mercury’s micro atmosphere
  2. New insights into detection thresholds for the Kepler Space Telescope. During previous transits of Mercury, no measurable abatement in solar irradiance was detected (see discussion above about the “Transit” method of exoplanet detection). With this transit, it is hoped that any measurable drop in solar irradiance will provide fresh insights into new detection methods and refinements to existing methods.

Imagination is more important than knowledge 585px-Albert_Einstein_signature_1934(invert)
An index of all articles in this blog can be found here.


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