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Oct 22 - 26

Transit of Mercury
November 11, 2019

On Monday, 2019 November 11, Mercury will transit the Sun for the first time since 2016. 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. There are approximately 13 transits of Mercury each century. In comparison, transits of Venus occur in pairs with more than a century separating each pair.

The RRAS will provide public observing of this event at our Rainbow Park Observatory.  The transit will begin before sunrise in Rio Rancho, and the sun should clear the houses to the east of the observatory by 8:00 AM.  The transit ends at 11:04 AM as seen from Rio Rancho

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 silhouetted 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. Position angles for Mercury at each contact are measured counterclockwise from the north point on the Sun's disk.

Geocentric Phases of the 2019 Transit of Mercury
Event Mountain Standard Time Position Angle
Contact I 05:35:27 AM 109.8°
Contact II 05:37:08 AM 109.8°
Sunrise @ Rio Rancho 06:37 AM  
Greatest Transit 08:19:48 AM 24.3°
Contact III 11:02:33 AM 298.8°
Contact IV 11:04:14 AM 298.7°

The table above gives the times of major events during the 2019 transit in MST. Greatest transit is the instant when Mercury passes closest to the Sun's center (i.e., minimum separation). At this time, the geocentric angular distance between the centers of Mercury and the Sun will be 75.9 arc-seconds. The position angle is the direction of Mercury with respect to the center of the Sun's disk as measured counterclockwise from the celestial north point on the Sun.

Observing the Transit

Since Mercury is only 1/194 of the Sun's apparent diameter, a telescope with a magnification of 50x or more is recommended 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. Amateurs can make a useful contribution by timing the four contacts at ingress and egress. Observing techniques and timing equipment are similar to those used for lunar occultations. Since poor seeing often increases the uncertainty in contact timings, an estimate of the possible error associated with each time should be included.

White light observations of Contacts I and IV include a small bias since Mercury is only visible after Contact I and before Contact IV. However, if Hydrogen-alpha filtration is available, the planet may be visible against either prominences or the chromosphere before and after Contacts I and IV respectively.

Observations of Contacts II and III also require amplification. They're actually the instants when the planet appears internally tangent to the Sun. However, just after the real 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 was sometimes called Contact II, but really is delayed by some seconds, up to a minute. Contact III occurs in exactly the reverse order. Atmospheric seeing often makes it difficult to measure contact timings with a precision better than several tens of seconds.