# Launch windows for Venus transit; how to convert heliocentric longitudes to date?

Being a bit familiar with celestial mechanics, I know that the Hohmann transfer orbit is the quickest way to transfer between two circular orbits of different radii around a central body in the same plane. JPL's Let's Go to Mars! Calculating Launch Windows explains the geometry and offers a very intuitive guide how to calculate the travel time to Mars, and this can easily be repeated for Venus.

Just for fun, I would like to mark all launch windows for a travel to Venus on a calendar, similar to the list compiled by Don P. Mitchell

1991, Jun 5     Unused Type I window
1993, Jan 5     Unused Type I window
1994, Aug 9     Unused Type I window
1996, Mar 20    Unused Type I window
1997, Oct 15    Cassini flyby


Question: I have a probably minor (astronomical) issue: I am struggling how to convert heliocentric longitudes into real dates? I guess I am missing the English terms here. I would appreciate a small reminder how to proceed.

• I think the "heliocentric longitudes to date" question is an astronomical question, but the other parts would be much better asked on the "Space" stackexchange, where you can find real rocket scientists. – James K Jan 1 at 22:56
• @JamesK I edited out the part which belongs to another site. – B--rian Jan 1 at 23:03
• With your edits and the splitting off of the other part to Space SE this question now seems on-topic here. – uhoh Jan 2 at 1:26