Elements & procedures from Jean Meeus, Transits (1989)
JavaScript & VML code by Gian Pietro Basello

Transit:    Bearing: north up zenith up
Latitude: ° ' "   Height: m      Toggle display: geo loc instant
Longitude: ° ' "   Put coordinates:
Instant (UT): h m s = h.dec    Toggle: geo loc    Realtime: 30s

geocentriclocalinstant
timePAtimePASAsunplanet
ingress
exterior contact		 TD
 UT		x:
ingress
interior contact		 TD
 UT		y:
maximum
(least distance)		 TD
 UT		()()d: d1:
egress
interior contact		 TD
 UT		M: M1:
egress
exterior contact		 TD
 UT		m: PA:
sun SD
planet SD
PA: position angle
SD: semidiameter
SA: sun's altitude

The large circle in the figure represents the limb of the solar disk. Five positions of the planet (the small circles) are shown, namely (from left to right) at the exterior and interior contacts at ingress, at the least distance to the center of the solar disk, and at the interior and exterior contacts at egress. N gives the direction of the North on the celestial sphere (not the direction of the observer's zenith); this corresponds to position angle P = 0°. The West (W) point corresponds to position angle P = 270°. The position angle is the angle measured counter-clockwise along the solar limb, from N through E, S, W. [from Meeus]

The five positions are shown in gray for an ipotetic geocentric observer and in black for the observer at the selected longitude and latitude (the small difference in position is due to the effect of parallax). You can enter the coordinates manually or selecting the location from the predefined list.

Entering an instant time (either in hours, minutes and second or hours and decimals of hour), the position of the planet is displayed as a red circle on the figure for the given Universal Time. You can toggle between the instant position for the geocentric or local observer. Checking the realtime checkbox, the instant time will be synchronized with your computer clock (this may be useful during the transit).

Procedures are implented in JavaScript: you do not need to reload this page and you can save and use it offline. Since you can view the code, I am aware that it is not too much elegant! The chart is implemented in Vector Markup Language, supported only by Microsoft Internet Explorer v.>=5.0. In the next days I hope to add a real-time control for the instant display.
(29/V/2004) VML chart code rewritten (3 hours work) and real-time added (5 minutes work).
(1-2/VI/2004) Rewritten refresh code: now there is one multi-purpose refresh function called from whatever input field changed.
(7/VI/2004) Zenith up.

©2004 Copyright by Gian Pietro Basello
for ElamIT.net <www.elamit.net> (old URL: http://digilander.libero.it/elam)
Write to <elam@elamit.net>

Napoli, 25-26.29-30/V/2004