Transit of Mercury on 9th May 2016

Public Observation Session on Transit of Mercury on 9th May 2016 at Arthur C Clarke Institute (16:30 onwards)

On Monday, 2016 May 09, Mercury will go across the Sun and Sri Lankan observers will be able to view this rare event, 13 years after its previous occurrence in 2003. The transit or passage of a planet across the face of the Sun is a relatively rare occurrence. As seen from Earth, only transits of Mercury and Venus are possible.

Observing the Transit

Never view the Mercury Transit with Naked Eye

Since Mercury is only 1/158 of the Sun’s apparent diameter, a telescope should be used with a magnification to watch this event. The telescope must be suitably equipped with adequate filtration to ensure safe solar viewing. The visual and photographic requirements for the transit are identical to those for observing sunspots and partial solar eclipses.

The principal events occurring during a transit are conveniently characterized by contacts, analogous to the contacts of an annular solar eclipse. The transit begins with contact I, which is the instant when the planet’s disk is externally tangent to the Sun. Shortly after contact I, the planet can be seen as a small notch along the solar limb. The entire disk of the planet is first seen at contact II when the planet is internally tangent to the Sun. During the next several hours, the planet slowly traverses the brilliant solar disk. At contact III, the planet reaches the opposite limb and once again is internally tangent to the Sun. Finally, the transit ends at contact IV when the planet’s limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress.

The following image shows the Mercury’s path through the Solar disk during the transit. We can observe the beginning of the transit (Contact I and II) but not the greatest transit as the sun sets around 18:16 local time. The timings of the transit are in the following table.

Contact PointsUT Time (hh:mm:ss)Sri Lankan time (hh:mm:ss)
Contact I (Start) 11:12:1916:42:19
Contact II 11:15:3116:45:31
End of the transit (Sri Lanka) 18:16:00
Greatest14:57:2620:27:26
Contact III18:39:1400:09:14
Contact IV18:42:2600:12:26

Transit of Mercury -Image taken at ACCIMT in 2003

White light observations of contacts I and IV include a small bias since Mercury is only visible after contact I and before contact IV. Using the Hydrogen-alpha filter at ACCIMT, the planet may be visible against either prominences or the chromosphere before contact I. Observations of contacts II and III also require amplification. They’re often mistaken for the instant when the planet appears internally tangent to the Sun. However, just before contact II, the so-called black drop effect is seen. At that time, the transiting planet seems to be attached to the Sun’s limb by a thin column or thread. When the thread breaks and the planet is completely surrounded by sunlight, this marks the true instant of contact II. Contact III occurs in exactly the reverse order. Atmospheric seeing often makes it difficult to measure contact timings with a precision better than several seconds.