Transits of Mercury and Venus

When the planets Mercury or Venus pass across the face of the Sun, the event is called a transit. These rare events are typically visible from large areas of the Earth. In the past when such a transit occurred The Astronomical Almanac would publish a table of phenomena that could be observed during the transit. For reasons of space, the table was limited to only a few select locations worldwide. Beginning with the 2012 transit of Venus, The Astronomical Almanac Online will also include such a table, but for many more locations.

For transits of Mercury and Venus there are five phenomena that are of interest: the four "contact points" and the Least Angular Distance. The first contact point occurs when the edge of the planet's disk first contacts the edge of the Sun's disk as it starts to pass between the Earth and the Sun (Ingress: Exterior Contact). The second contact point occurs when planet's disk has crossed the edge of the Sun's disk and is last in contact with that edge (Ingress: Interior Contact). The full disk of the planet, which appears as a dark shadow, is now within the illuminated disk of the Sun. The fourth and fifth contact points are the reverse of the first two. The fourth contact point occurs when the edge of the planet's disk contacts the opposite edge of the Sun's disk from where it entered (Egress: Interior Contact) and the fifth contact point is when the edge of the planet's disk is last in contact with the Sun's disk (Egress: Exterior Contact). The transit ends at the time of this last contact. Associated with these contact points are position angles that help locate the planet as it crosses the Sun's disk. The position angle is measured in degrees from the apparent north pole of the Sun eastward towards a line from the Sun's center to the center of the planet. Along with the position angle, a direct measurement of the apparent distance between the center of the planet's disk and the center of the Sun's disk is made. This distance is given in minutes and seconds of arc. The Least Angular Distance is the point of apparent closest approach between the center of the planet's disk and the center of the Sun's disk. The accompanying diagram illustrates all of these quantities.

The table of phenomena presented below gives the times and position angles associated with the four contact points and the Least Angular Distance for each Earth location included. All of the times of the phenomena in the table are expressed in Universal Time (UT) using a value for Δ T given in the table.

In selecting locations to include in this expanded table, one factor considered was one of the same used in selecting locations for the table included in The Astronomical Almanac: two of the five phenomena must be observable from the location. Another factor considered in choosing locations was population. In countries where multiple locations exist, those with the highest populations were selected first with locations with slightly smaller populations considered next to insure good coverage in a region. For islands, especially those in the Pacific Ocean, the locations chosen had the highest populations, although their populations often were much smaller than locations chosen on the continents.

The Astronomical Almanac Online! 2020