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Heliacal Setting experiment


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As is well known many cultures have attached importance to the "heliacal" rising and settings of stars, when they are first visible before sunrise in the morning and last visible after sunset in the evening. Sirius and Egypt is the best known, but I also know of others (e.g. Matariki in New Zealand, based on the heliacal rising of the Pleiades). But what's it like trying to observe these events? I've always been interested to try and find out.


So how dark does the sky need to be, and how high the star? And how accurate are the various calculations you see online for them? Calsky (a great program) gives heliacal rise and set dates for your location, if you go to the "deep sky" section and click on the star map. Many of them seem to tally more or less with my observations of how early/late I can see particular stars, though I'm doubtful of others (18 September for the heliacal setting of Antares seems too early). Last night and tonight I went to try and do more thorough testing...

I have a fairly good view to the W and NW, well placed for seeing stars at dusk. Last night I watched the stars come out and I could pick out Betelgeuse and Aldebaran, M45, Auriga and Perseus. Trying to see how low I could get I couldn't see any of Aries but could see gamma Andromedae easily, and low down could just see Beta (Mirach) deep in the twilight. This was about 21.20 (BST). I though it must be close to heliacal setting so checked Calsky and it gave 19 April as the date! However it stated it didn't become visible until after 2200, presumably as it was not (so they thought) dark enough.

Intrigued to see if I could beat Calsky I went out again tonight to see if I could see it after their heliacal setting date. As it got dark I watched the stars appear- Orion, Aldebaran, Capella then Cassiopeia and Perseus and gamma Andromedae. I had memorized where Mirach was yesterday and looked for it to come out. Sure enough, at 2122 it emerged from the twilight, very faint at first but I was able to hold it after 30 seconds or so. This is the star that Calsky had heliacally setting yesterday. It was about as easy to see as a mag 5 star overhead. I kept sight of it for a good 10 minutes before going in to check what Calsky had to say. It said "Binoculars Required" to see it this evening, again not till after 2200. Not so, Calsky!

When I first saw it its altitude (from Calsky again) was exactly 7 degrees, with the sun 9.1° below horizon- well within nautical twilight. At 2135 when I finished observing it was 5.86° with Sun 11° below. A waxing gibbous moon was in the sky, though a long away in the east. This is a mag 2.1 star, so much fainter than Sirius. 

I will try and see what I can do with Sirius, Betelgeuse and Aldebaran in the next couple of weeks- Calsky has their heliacal settings in late April/early May.

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Tonight offered another opportunity, focusing this time on the winter first magnitude stars. 

Betelgeuse is still well up as darkness falls, it came into view about 2115 and is still just visible now. Probably has a couple of weeks left. Procyon even more so, I've seen that in May easily.

At 2125 I found a spot with a clear view to the west and was rewarded with the clear sight of Sirius glowing low in the still quite intense twilight. It's still with us! It was low, but still very conspicuous. The altitude was 4.3°, Sun -7.8°. As it punches through a lot of twilight I think the combination of a cloudless horizon, clear view and transparency will be essential to how late it can be seen. Calsky says 1st May. 

Now for a surprise: It was much harder, but I could make out the three stars of Orion's Belt. Slightly higher than Sirius but much fainter, and closer to the point of sunset, you had to know where to look (using Sirius, Betelgeuse and Bellatrix- also visible- as reference points) but they were all still visible. Now for Mintaka, Calsky has the heliacal setting four days ago, April 23! It says I should see it at 2129, "binoculars required". But I saw it without them, at exactly that time. Star altitude 7.2°, Sun -8.2°, mag +2.2. So more marginal than Mirach last week. Alnilam (which was the first one I spotted) it has HS on April 26, yesterday. "Binoculars" again, or not. Alnitak- also April 26 and binoculars required. Calsky beaten on all of them. No doubt it helps that Mintaka is so close to the other two, if it were an isolated star it could well have been much harder to locate.

Rigel and Saiph are well gone though. At that time when these stars appeared Rigel has already set, and Saiph only 2° high. I have never seen either after mid-April, Calsky's April 17 for Saiph and April 21 for Rigel seems about right.

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  • 2 months later...

With the winter stars long gone, now it's the spring ones. Jupiter is still with us, a respectful glance at him with the binoculars just now yielding a moon despite the twilight. Denebola is still well up as darkness falls, Algieba lower but still there. But what about Regulus? Saw it on Sunday evening setting into the twilight, and this evening I couldn't find it with the naked eye around 2240, so looked with the binoculars after seeing Jupiter. I found it literally perched on a distant rooftop, it was easy to see in the binoculars. Could I see it naked eye though? Yes I could, by moving backwards through the door (which faces west) so it edged above the roof. It was there, but I wouldn't have seen it without using the bins first as it was only just beating the twilight. Its altitude was just under 6°, Sun -8°. So a similar Sun altitude to when I saw the second magnitude stars in the previous posts, but a lower star altitude. Higher than Sirius though. Calsky's HS date for Regulus: July 2. 

With sunset times more or less static at this time of year, the stars are disappearing much more slowly than in April. After late July the sun starts setting noticeably earlier, any stars still lingering in the west will have their HS delayed longer. Denebola doesn't disappear until 13 August according to Calsky, that's one to watch.

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  • 2 weeks later...

Another look tonight, and it's safe to say Regulus has gone. Denebola and Zosma are still easily visible though, no more difficult than a couple of weeks ago. The tricky one tonight was Chertan (theta Leo) which at mag 3.3 is the faintest one attempted so far. At 22.55, alt 9.6 Sun-10.2, it was there but very difficult; faint with averted vision and barely visible directly, could be held for about 5 seconds at a time. This was in still fairly bright twilight with a full moon in the opposite part of the sky.

Calsky's HS date for Chertan? June 25. Way off that one, as it remained visible past the solstice it did not disappear rapidly and has survived into the lengthening nights. It seems the limit for a fainter 3rd mag star is about 10/-10 degrees.

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  • 2 months later...

Time to get back to this at last:-)

Tonight I can confirm what I thought- Antares is visible in October given the right conditions. Calsky had it HS on 23 September (last year it was 18th, it's that marginal that the equinox date seems to affect it), but as it I'd disappearing as sunset is falling earlier it's a cat and mouse game. At 1935-1940 I spotted it, directly under Saturn, appearing as about a mag. 3 star through the twilight (despite a bit of horizon haze around). It was around 4° high, with Sun around -8°.

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