aka Calculating the heliacal rising of Sirius
What is a heliacal rising and why are we calculating it?
The heliacal rising of a star is when the star rises at dawn and is visible for a few minutes before sunrise (at which point the brightness of the sun makes it invisible again). Since this date varies based on location (specifically latitude), you’ll need to look up the rising of Sirius at your location to find out what date it falls on where you live.
(Or you can pick a date during July or August; that’s a valid way of doing it too! But since you want to know how to calculate it…)
Look up your city’s coordinates on wikipedia or google. You’ll get something like this: 43°4′N 89°24′W. You’d read that as forty-three degrees and four minutes north, eighty-nine degrees and twenty-four minutes west. Sometimes they say minutes and seconds, sometimes just minutes. Seconds don’t seem to affect this calculation, so don’t worry about that.
Once you have your coordinates, open this link in two tabs:
On the first tab, select Sirius, on the second one, select Sun. For both, enter your latitude, longitude, and time zone.
For the date fields, it’s easier if you already have a general idea of what date-range you’re looking for, so check my cheat sheet to narrow down your date-range. For 43°N, it should be on or close to August 12th, so let’s put in August 10 for a start date, and leave the other fields as is.
(I’m doing this in “Locations Worldwide” so everyone can see how to do it, but if you’re in the United States you can use the shorter form at the top.)
The screenshot above highlights the parts that you have to fill in. The form for Sun is identical, except it has Sun selected instead of Sirius. Hit “compute” and you get a list of times. You only need the first column of each.
Let’s look at the results:
On Aug 12, Sirius rises at 04:23, and dawn (aka begin civil twilight) is at 04:29. We are looking for when Sirius rises at or just before dawn. This is a few minutes before dawn, but let’s see if we can go even earlier:
On Aug 11, Sirius rises at 04:27, and dawn is at 04:28. Sirius is rising just before dawn. This looks like it will be the one.
Can we go even earlier? No, because on Aug 10, Sirius rises quite a bit after dawn. (Sirius rises at 04:31, and dawn was at 04:26.)
Therefore, August 11th is the first day that Sirius rises at or just before dawn. That is your heliacal rising of Sirius, Lokabrenna, or Wep Ronpet. 🙂
I hope that makes sense. Please let me know if I can make anything clearer.
- The form asks for longitude followed by latitude, whereas coordinates are given as latitude (N/S) followed by longitude (W/E); make sure you’re entering your numbers on the correct line.
- The defaults on the form are north and west. If your coordinates are south and/or east, remember to select that.
- If you forget to put in your time zone, your numbers will still be correct, they’ll just be given in UTC time instead of local time.
Credit where credit is due, I got my information from this page. Since I’m more of a visual learner, I wanted to make a walkthrough with pictures so it’s easier to follow along, but it’s basically the same info in a new format.