"Sunset Calculator"

Practical astronomy by Tore Aasli

The sunset calculator is a simple astronomical calculator for personal home use.

The idea behind this calculator was born a couple of years ago, as I missed an instrument to tell time and dates for sunrise, sunsets, lunar phases and more.

Today I use this calculator in my leisure activities and as a sailor's tool.

Here is a screenshot of the "sunset calculator":


Using the calculator

Firstly, you have to register your favourite Places with the + button, together with their latitudes and longitudes (in degrees, minutes and seconds). If you have several places in your database, click on one of them to activate it. Click on the v (check mark) button to save it to the database.

Then, the date must be filled in. When the program starts, today's date is the default but you can change this to whatever date you like, either by entering the date manually in the date edit box, or by using the date picker above this edit box.

Then, click on the Compute button to have the calculator to fill in the sun's data, twilight data and moon data for this date.



Easter Sunday

Interesting! Easter Sunday is defined to be the first Sunday after the first full moon after the March equinox. So, the time and date of full moon should be (just) passed.

Try this: Click on Compute for a given year (or this year). Use the Datetime picker to select the Easter Sunday date (or fill in Easter Sunday date in the Date editbox manually). Click on Compute again. Now take a look at the moon image, it should show a waning moon since the full moon has already passed.


Twilight is the periodes just before sunrise and after sunset. There are three definitions of twilight:

  1. Civil Twilight
  2. Nautical Twilight
  3. Astronomical Twilight

I stick to Civil Twilight, because for 2) and 3) the sun is quite low under the horizon. Civil twilight gives off "more light" than 2) or 3) and might be just suitable for reading a map or nautical chart.

Next solar Eclipse

This date and time is focused only on a partial or full solar eclipse, and the dates / times are for your selected latitude and longitude in the database. The label under the solar eclipse date and time explains if it is a partial or full eclipse.

Next Blue Moon

One calendar month has usually one full moon. Occasionally there are two full moons in a calendar month, and the second full moon is what I call a Blue Moon. This occation happens quite rarely, hence the saying "somthing happens once in a blue moon" about rare events, like a Lotto winning.

There is also another definition of a blue moon, which might be more correct, but I stick the most common definition in my program as mentioned above.

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