This page computes the local circumstances for any transit of Venus between 1639 to 2117. You can change the location by either dragging the marker on the map, searching for an address or entering the longitude and latitude. When entering the longitude and latitude in degrees and decimals, please consider east longitudes and north latitudes as positive. The height is measured in meters above sealevel.
Notes on the data
Planetary transits start when the planet’s disk is externally tangent with the sun (ingress, exterior). From then, the planet may be discerened as a little black dent in the solar limb, gradually growing bigger until the entire planet is seen on the solar disk (ingress, interior). During the next five to six hours, the planet will traverse the sun’s disk until the planet’s disk will touch the opposite solar limb (egress, interior). The transit ends when the planet’s disk is externally tangent with the sun (egress, exterior).
For all four contacts, the time, the position angle of Venus, the angular distance to the sun’s centre and the sun’s altitude and azimuth are given. If the sun’s altitude is negative, this means that the sun is below the horizon and, subsequently, the particular contact is invisible from the specified location. This is indicated by an asterix. The azimuth is the compass direction, measured eastward from the north. In addition, the approximate times of setting and rising of the sun are given. Instead of sunrise and sunset, you can choose to show the start and end of more ideal observing conditions, when the sun is at least 8° above the horizon.
You can compute the local circumstances for any instant between ingress and egress using the slider. Not only are the position angle, angular distance and the sun’s altitude and azimuth updated when you move the slider back and forth, the position of Venus on the solar disk at the selected instant is also indicated in the diagram right to the map.
The time zone offset and daylight saving time are selected automatically, but you can change these if you want to. If you check local time, the local mean time is given, which is mean solar time reckoned from midnight. Because in history astronomers usually kept the solar time of the local meridian, this option may be convenient if you want to compare predicted times of contact with ones observed in the past. To get apparent (or true) local time, check the equation of time, which is also given in minutes and seconds. When consulting historical sources, you should observe that the hours and days were often reckoned from noon. If this is the case, substract twelve hours from the times tabulated above.
The diagram next to the map shows the movement of Venus across the solar disk relative to the zenith, the point directly overhead. This is how Venus advances on the solar disk when observing with the naked eye or a telescope on an altazimuth mounting. Because of the diurnal motion of the celestial sphere, the sun’s disk rotates with respect to the direction of the zenith in the course of a day. The initially straight chord, representing Venus’ trajectory with respect to the north point, is now transformed into a curved path. The gray disks of Venus indicate the invisible part of the transit during which the sun is below the horizon. The line along the solar disk indicates the direction of the celestial north.
Copyright and acknowledgments
All text, maps, diagrams and calculations are made by Steven van Roode, unless stated otherwise. The times of contact are computed using the iterative algorithm and Besselian elements provided by Jean Meeus in his book Transits (Willmann-Bell, 1989). The Javascript code is based on a script by Franco Martinelli. The localized geographic content is generated by MaxMind. The height, time zone offset and daylight saving time data are obtained from Geonames.