Measurement of Maximum Elongation of Venus

The section on the simple understanding of AU determination using transits, talks of the need for knowing the maximum elongation of Venus, for this determination - in a simplified approximation of AU determination using transit timings.

Elongation of any celestial body refers to its angular distance from the Sun, as seen from Earth. Planets have the least elongation when they are at conjunction with the Sun and the maximum elongation of 180⁰ when they are at opposition. However, the planets we are interested in, Mercury and Venus, the only planets that could transit in front of the Sun, do not ever reach opposition. From the time of conjunction, they move away from the Sun until they reach a maximum value of their elongation, and return towards the Sun. Mercury reaches a maximum elongation of about 28⁰ from the Sun, while the maximum elongation for Venus lies within 45⁰-47⁰, with a small jitter of change seen from year to year, arising from elliptical orbits.

The orbit of Venus, is almost circular, compared to that of Mercury and the jitter seen in the figure above, has contributions mainly from the changing Earth-Sun distance, at each maximum elongation.

  In the year 2004, for instance, Venus starts as a bright evening object in the south western sky, in the constellation of Capricornus – easy to identify as it rules the skies – the brightest object other than the Moon (when visible), following Sunset. In the months following that, it goes through increasing elongation and appears higher in the sky following sunset, going through the constellations of Aquarius, Pisces, Aries and reaching Taurus, by April.

It will have company from Mars, in this month. Before that, on the 29^th of March, it will have its maximum eastern elongation and start falling back towards the Sun, again. After going through inferior conjunction and transit on the 8^th of June, its elongation will increase to go through another maximum elongation in the month of August. Its phases also change accordingly, decreasing from a maximum Full Phase, through the months of January-June, passing through the New phase on the day of the transit and increasing again.

It will be an exciting activity in the last week of March and the first week of April 2004, or, at other suitable times – August 2004, and again, March and August 2012, for students to directly map the changing elongation of Venus. Some methods for this can be found here.

A method for measuring the maximum elongation of Venus, using the Jayaprakash Yantra and Jantar Mantar  Delhi and Jaipur, will be uploaded here, soon.

Transit of Venus homepage