Algol Minima Predictions
Prepared by Joe Stieber, Sept 2022



S&T Online Algol Minima Calculator, early October 2022

This is a screen capture of the Algol minima calculator from Sky & Telescope online. It shows the next set of observable minima coming up in October 2022 for the New Jersey observer. As discussed below, the online calculator has demonstrated its reliability by recent observations, while in contrast, the table of minima listed in Sky & Telescope magazine has been found to be unreliable. Click here for a printable version of the S&T magnitude reference chart. Here's some more info about Algol.




S&T Magazine, Sept 2022

This is a screen capture of the Algol minima table on page 50 of the September 2022 issue of Sky & Telescope magazine (the digital edition rather than the paper version). Note that Algol's period is given as 2.867351 days below the timing table, but 2.7 days under the magnitude reference chart (I have no explanation for this discrepancy). I also checked the minima times with S&T's online calculator, which provided significantly different predictions as shown below (and I added the "Not Accurate" overlay to the table above).



S&T Online Algol Minima Calculator, Sept 2022

This table is a screen clip of Sky & Telescope's online The Minima of Algol calculator, using a starting date of Sept 6, 2022. Note that for the minimum on Sept 17, the time is Sept 18 at 1:31 UT, or Sept 17 at 9:31 pm EDT. That's 20 hr 19 min later than the magazines prediction! For further reference, I checked the predictions offered by SkyTools software, versions 3 & 4 (both use the same 2.8673 day period, slightly different than the 2.867351 day period listed under the magazine's table). ST4 predicted a minimum on Sept 17 at 7:58 pm EDT (1:33 before the S&T calculator) and ST3 predicted Sept 17 at 9:01 pm EDT (0:30 before the S&T calculator). The SkyTools predictions were in far better accord with the online calculator than the magazine's prediction.

To establish some "ground truth" about the actual time of the minimum, Suzanne Leap (WAS Secretary & Star Watch Director) and I observed Algol on the night of Sept 14-15, 2022, between 11 pm and 3 am EDT from a suburban location in Haddonfield, NJ, under a nominal 75% illuminated Moon about 22.5 from Algol. It appeared that Algol was dimmest around 1:30 to 2:00 am, and it had definitely brightened by 3 am when we left. So, it would appear that the online calculator prediction is at least reasonably close to the true minimum.

Update Sept 17, am: The magazine predicted a minimum on Friday night, Sept 16-17, at 1:12 am EDT, so I went to Collins Lane Park in Maple Shade, NJ, for a quick check. At 1:07 am, Algol was evident with unaided eyes, a bit dimmer than mag 1.8 Mirfak, while nearby mag 3.65 Rho Per (variable, mag 3.3  to 4.0) was barely visible. With 8x42 binoculars, Algol was significantly brighter than Rho in contrast to Algol at minimum being only slightly brighter than Rho.

Update Sept 17, pm: During the WAS Public Star Watch at Batsto, Algol was observed with unaided eyes and binoculars. Around 9:30 pm EDT, it was clearly dim, not much different from Rho Per, so one had to pay attention to pick it out. It gradually brightened and by 11:30 pm, it was obvious with unaided eyes.

So it appears that the predictions in the September issue of S&T are simply wrong, and not by a small amount. They're an average of 20 hr 18.5 min early vs. the online calculator, or about two minima cycles, which are nominally 10 hr wide from the start of dimming until it returns to full brightness ( 5 hr from the minimum). The same erroneous minima are listed for September in the Algol table of the October 2022 issue, also pg 50. If you intend to observe Algol's brightness variations, ignore the S&T magazine predictions.



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