Looks like the link I had to that French Sirius calculator has died. Dang it! So I’ve looked around and have something to offer as a substitute. It’s not as easy to interpret, but here’s what we’ve got:
Once you go to the site, input the latitude of your location or the location for which you want to know the heliacal rising. Then enter a date…probably sometime in July or August. Note that the farther north your latitude, the later Sirius rises.
Then look at the timeline below the animation. Look for the blue “star above horizon” line. As you change the date with the red slider (upper right), you’ll see that line move. You’ll also see the darkness-to-light transition on the timeline. You will probably be able to see the star more easily about an hour before dawn. So set the red line in the timeline at about an hour before dawn (dawn is where the grey turns to yellow in the timeline).
Keep messing with the date until you see the “star above horizon” blue line at about an hour before dawn. If you have a perfectly clear morning—and all the Goddesses and Gods are willing—that’s when you might be able to see Sirius rise in your area. Or it could be a couple of days later when you’ll actually be able to observe it with your naked, non-mathematical eyeballs.
Celebrating Isis’ birthday on any of the dates around the rise of Sirius in your area will please the Goddess just fine. Luckily for me, August 17-18 is the mathematical rising in my area, but I probably wouldn’t be able to see it until the 20 or so. But since the mathematical date is near a weekend…I will probably celebrate Her birthday on the 18th or 19th—and that will be well within bounds.
See what you figure out for your area…and have a happy Isis’ birthday!