webspotting 10 - twilight

First published in the Jun/Jul 2009 issue of SCOPE, the newsletter of the RASC Toronto Centre. Very minor edits applied. Republished here with permission.

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When I'm preparing for an astronomical observing session, I like to know when sunset will be and, of course, when it will get really dark. I'm not usually worried about sunrise, being a nyctophiliac.
 
Like many budding amateur astronomers, as I got started in this curious hobby, I did not know there were 3 flavours of twilight! If you're still a little fuzzy on details, check your Observer's Handbook or The Amazing Wikipedia, of course.

http://en.wikipedia.org/wiki/Twilight

Now, to the matter at hand. You may need to know sunset and sunrise times, if you're chasing some inner system planets. Perhaps you want to know when astronomical twilight will begin and end, in preparation for an all-night deep sky photography session. Or you're searching for elusive daytime planets, working your spherical trig, and need to know when the Sun will transit.

http://SpectralCalc.com/

Jump into the atmospheric science web site by GATS, Inc. and click on the Solar Calculator.

Set your location. At the bottom of the drop-down menu, below Washington DC, you’ll find World Cities. If you're doing rough estimates, you can select Ottawa. Or Saskatoon. No Toronto listed. I thought we were The Centre Of The Known Universe; I guess it's Nauru Island.

If you're entering your exact coordinates, be mindful of our western longitude. Enter this as a negative number.

Similarly, you must enter the timezone offset as a negative value. They have the Ottawa one incorrectly stored as a positive number; Saskatoon is correct. Then remember to add one (positive) to adjust for DST.

So, for the High Park City Observing Session in April, I used latitude 43.650222 and longitude -79.467013 and timezone offset -4.

Choose your preferred date.

Then hit the Calculate button. The JavaScript will quickly churn your data.

A table will be shown at the bottom of the web page. The "Begin" numbers refer to dawn; the "End" numbers refer to dusk.

If you want to compare locations, do not click the Clear Results button. Simply change the form data and Calculate again. Another table will be added to the page.