Online Astro Calculator

While you are at it Dave, can you write one for transits of the ISS across the disc of the Moon and Sun as visible from Ireland? Should be a *simple* task for someone like yourself.

Would you beleive I'm on that one. I've already written code to calculate a satellites XYZ vectors as per NORADs SGP4 algorithm. I've done the bit to transform these into alt/az co-ordinates for the ground viewer. Where I'm at now is finding an effecient way to calculate actual pass timings. This involves a lot of calculus and I was never brill at calculus. I've also been playing with Ed Moran's java ISS transit code and made some interesting mods. I removed much of the restrictions such as 160km radius and 11 day limit. The problem I see with turning all this into a web app is it involves a huge amount of processing. That's why Ed Morana (I think) used a client side java applet, to push the processing burden to the users PC.

Looks good DG, any plans to share the code or even the algorithms used? I'd be pretty interested in seeing whats involved in the calculation.

Albert, the algorithm used is described in the book 'Astronomical Algorithms' by Jean Meeus (Willmann-bell ISBN 0-943396-35-2). The algorithm used here is described on pp 277-283. For anyone interested in such calculations this is 'The Bible'. As for the source code, it is so highly custom it would be pretty meaningless in many ways. When I'm done with all this I might be able to publish the PHP class module which is in effect the heart of the calculations. The problem is unless you know what your looking at even that would be hard to use.

A better approach might be to compile all my work into a dll class which could be used in your favourite spreadsheet or other app.

Hi Dave,
I found the passwords a bit difficult to read tho' (maybe I need my glasses changed).

Me too, however lookin g at that again may have to wait. I found that you never have to click the word refresh button more than once or twice max to get one thats easy to read. My intention is that thiese types of app are used not that frequently, so maybe this isn't such a bad thing.

Here is another idea,
How about io/europa/ganymede/callisto transit times along with shadow transit times, now that would make it even more useful.

Another one in progress. This is obviously a completely different app to the GRS transit calculation. Nonetheless, I have written code to calculate the co-ordinates of the 4 moons. Its on my list to put this into a format to provide event timings.

Also, I am sure you can add a line of code to read the date on one's own computer everytime that applet loads

Excellent idea and I can make it so the 'to' date is 7 days ahead thus including Dave Lillis's idea in one swoop.

Albert, the algorithm used is described in the book 'Astronomical Algorithms' by Jean Meeus (Willmann-bell ISBN 0-943396-35-2).

Ah... I've been looking for a copy of that for years (at a reasonable price!) but never managed to pick it up. I've read some software modules/classes based on the book but its hard to understand the reasoning behing the calculations without the book.

I do have Practical Astronomy with your Calculator which is good, but does a very poor job of explaining how the algorithms work too!