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PostPosted: Tue May 12, 2020 11:57 am 
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You may want to look at the Moon, Saturn, and Jupiter close together an hour before sunrise for the remainder of this month. Read all about it here:

https://www.thrillist.com/news/nation/h ... y-may-2020

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PostPosted: Tue May 12, 2020 2:28 pm 
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I've been incidentally following the Moon, Jupiter, Saturn and Mars when I've been out early the past four mornings in an effort to spot comet C/2020 F8 (SWAN) around 4:30 am EDT (roughly half an hour after the start of astronomical twilight and an hour-and-a-quarter before sunrise). Because of the Moon's presence, the start of twilight isn't so obvious, it's around 4:45 am when there's a distinct glow above the eastern horizon.

Anyway, the 72% illuminated moon was about 3° below Jupiter this morning (Tuesday, 12-May-2020), forming its tightest grouping with Jupiter and Saturn for this lunation. By tomorrow (Wednesday, May 13), the Moon will be 8° below-left of Saturn, and it will keep moving swiftly eastward, reaching New Moon on May 22. So, the closest grouping of the three has already passed, and the Moon will not be with Jupiter and Saturn for the remainder of the month.

And so far, I have not been able to detect C/2020 F8 (SWAN) with my 15x56 binoculars or my 88 mm apo spotting scope. Saturday and Sunday mornings (May 9 & 10), I just couldn't see it at the low altitude (nominally 3° or so) in a moonlit sky, and on Monday and Tuesday (May 11 & 12), there were clouds along the eastern horizon. To compound the situation, the hoped-for brightening of this comet, up to the magnitude 3 range when it reached our northern skies, has not materialized. It peaked at the high 4.x range in early May, then dropped and flattened out in the mid to high 5.x range. See the magnitude data at COBS...

https://cobs.si/analysis2?col=comet_id&id=1876&plot_type=0

However, hope springs eternal for the determined comet spotter, so I'll be back out tomorrow morning if the actual sky corresponds with the current Clear Sky Chart prediction. I also want to get my binoculars on Neptune, which is below the Water Jar of Aquarius now, about 20° east of Mars. I haven't seen it yet this apparition, and it would make four morning planets.

Finally, don't forget to look at Venus with a scope (or perhaps binoculars or even the unaided keen eye) in the evening to see the slim crescent getting slimmer. It's only a few weeks until inferior conjunction on June 3. Try to catch it soon after sunset when it's a bit higher in the sky the background is still relatively bright to temper the glare from this brilliant object.

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PostPosted: Mon Jun 08, 2020 7:46 pm 
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The Moon, Saturn, and Jupiter will be close together tonight, June 8 & 9, 2020. Saturn and Jupiter are close to each
other in the sky only once every twenty years. Read about it here:

https://news.cgtn.com/news/2020-06-08/D ... index.html

Thanks to Greg Barr for directing me to this webpage.

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PostPosted: Tue Jun 09, 2020 8:21 am 
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I was looking at the grouping of Saturn, Jupiter and the Moon the past two mornings, about 3 am EDT on both June 8 and 9, 2020. On June 8, they were on a lightly bent line with the Moon to the far right, covering an 11.5° span from Saturn on the left. On June 9, the Moon was below, and slightly left of Saturn forming an "L" shape. It was about 8° from the Moon to Jupiter on the right. The grouping was a bit more compact on June 9, but the increased proximity of the bright Moon to Saturn made it a little more difficult to see Saturn, by far the dimmest of the three objects. The Moon was nearly full on both mornings, 92% illuminated on June 8 and 86% on June 9, but the sky was quite clear in each case. In any case, a fine sight for unaided eyes both mornings!

As far as the convergence of Jupiter and Saturn every 20 years or so, there's a simple rule for calculating that time period. Take the inverse of the period for the more rapidly revolving planet, subtract the inverse of the period for the slower revolving planet, then take the inverse of the difference. For Jupiter and Saturn, the orbital periods are 11.862 yr and 29.457 yr respectively (per Wikipedia). So we have...

1/((1/11.862) - (1/29.457)) = 19.86 yr

As far as the Moon passing close to the current Jupiter & Saturn pairing, or any relatively fixed object in the sky, that happens roughly once a month. Because the Moon's period is so much shorter than Jupiter's or Saturn's, the rule above has little impact. I found empirically (using SkyTools) that the moon was passing those two planets every 655 hours (or 27.3 days) in recent and coming months. Then I checked the RASC Observer's Handbook and found that 27.3 days is a sidereal lunar month, which shouldn't be a surprise. We're just so used to the 29.5 day synodic month, but that measures the interval between phase recurrence, whereas the sidereal period measures the return to a point with respect to the background stars (or relatively stationary planets).

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