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Skynotes August 2021

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Skynotes August 2021 AUGUST from Hermanus 2021 1. SKY CHARTS EVENING SKY 6 t h AUGUST at 21h00 (NORTH DOWN) EVENING SKY 6 t h AUGUST at 21h00 (SOUTH DOWN) 1 2. THE SOLAR SYSTEM PLEASE NOTE: All events predicted are as observed from Hermanus, Western Cape, South Africa. Times are South African Standard Time (UTC +2). Also please note : with the exception of Pluto (magnitude +14.4), all events predicted are visible to the naked eye. HIGHLIGHTS FROM THE SKY GUIDE Date Tim e It em 1 16h00 Mercury at superior conjunction 2 09h36 Moon at apogee (404 410 Km) 07h24 Saturn at opposition 3 06h28 Moon passes 6.1º north of Aldebaran 04h51 Moon at ascending node 1 5 18h46 Moon northernmost 8 15h50 New Moon 11 After sunset A fine crescent Moon (11%) paired with Venus 13 Moon (29%) about 6º north of Spica (α Vir) 15 17h20 First quarter Moon 16 18h04 Moon at descending node 2 23h20 Moon passes 4.6º north of Antares 17 11h25 Moon at perigee (369 126 Km) 19 00h24 Moon southernmost (-25.8º) 20 01H05 Jupiter at opposition Uranus stationary 21 03h19 Moon passes 3.2º south-east of Saturn 22 14h02 Full Moon Before sunrise Moon near Jupiter 25 Winter solstice on Mars (southern hemisphere) 30 09h13 Last quarter Moon 04h23 Moon at apogee (404 098 Km) 07h13 Moon at ascending node 1 [from1 the glossary of the Sky Guide Africa South] 1 ASCENDING NODE – in the orbit of a solar system body, the point where the body crosses the ecliptic from south to north. 2 DESCENDING NODE - in the orbit of a solar system body, the point where the body crosses the ecliptic from north to south. Astronomers push for global debate on giant satellite swarms : https://www.nature.com/articles/d41586-021-01954-4 ... time is tight. SpaceX is launching fresh batches of Starlinks — around 60 satellites per batch, sometimes several times a month. “People are spending years establishing relationships, but in the meantime the satellites are launching continuously,” says Venkatesan. “It’s almost like we are arriving at solutions for a problem three years ago.” 2 1st 1st AUGUST 2021 Visibility August September Rises: 07h36 07h02 Never look at the Sun Cancer to Leo Transit: 12h49 12h43 sun without Length of 10:56 to 11:23 SUITABLE EYE day Sets: 18h03 18h25 PROTECTION! Mercury Cancer to Virgo Rises: 07h43 08h02 Magnitude +2.0 to -0.0 Transit: 12h51 14h10 Low in the west Phase 100% to 73% after sunset Diameter 5” to 6” Sets: 17h59 20h19 Venus Leo to Virgo Rises: 09h19 08h46 Magnitude -4.0 Transit: 14h59 115h10 Evening Phase 82% 73% Diameter 13” to 15” Sets: 20h40 21h35 Mars Leo Rises: 08h49 07h39 Low in the west Magnitude +1.8 Transit: 14h18 13h29 Phase 99% to 100% after sunset Diameter 4” Sets: 19h48 19h20 Jupiter Aquarius to Capricornus Rises: 19h31 17h10 Throughout the Magnitude -2.8 to -2.9 Transit: 02h12 23h51 Diameter 48” to 49” night Sets: 08h48 06h37 Saturn Capricornus Rises: 17h57 15h45 Magnitude +0.2 to +0.3 Transit: 00h56 22h41 Throughout the Diameter 19” to 18” night Sets: 07h50 05h42 Uranus Aries Rises: 01h35 23h30 Magnitude +5.8 to +5.7 Transit: 06h52 04h51 Morning Diameter 4” Sets: 12h09 10h08 Neptune Aquarius Rises: 21h23 19h17 Magnitude 7.8 Throughout the Transit: 03h39 01h35 Diameter 2” night Sets: 09h52 07h48 Pluto Sagittarius Rises: 16h42 14h37 Magnitude +14.3 Throughout the Transit: 23h50 21h46 night Sets: 07h02 04h58 Phase: In a telescope, the inner planets (Mercury, Venus and Mars) appear to us in phases, depending on the angle of the Sun’s illumination, as does the Moon. The angular diameter is given in arc seconds (“). This is the apparent size of the object as we see it from Earth. Magnitude : we are accustomed to hearing stars described in terms of ‘magnitude’. For example the planet Jupiter at magnitude -1.8 is considerably brighter than the star Antares (in Scorpius) at +1.05. The scale is ‘inverse’; the brighter the object, the lower the number . A ‘good’ human eye on a clear night can see down to a magnitude of about +6. Transit: When an object crosses the local meridian it is said to ‘transit’ . The local meridian is an imaginary line from the horizon directly north passing overhead (through zenith, see charts on page 1) to the horizon directly south. 3 THE MOON . Lunar Libration In lunar astronomy, libration is the wagging or wavering of the Moon perceived by Earth-bound observers and caused by changes in their perspective. It permits an observer to see slightly different hemispheres of the surface at different times. It is similar in both cause and effect to the changes in the Moon's apparent size due to changes in distance. It is caused by the three mechanisms detailed below, two of which cause a relatively tiny physical libration via tidal forces exerted by the Earth. Such true librations are also known for other moons with locked rotation. The images to right show an example of libration. Note the different positions of the features Tycho and Imbrium . The black line represents a common axis which, itself, librates. The Moon keeps one hemisphere of itself facing the Earth (tidal locking). The first view of the far side of the Moon was not possible until the Soviet probe Luna 3 reached the Moon on 7th October 1959 followed by further lunar exploration by the United States and the Soviet Union. Over time, thanks to libration, about 59% of the Moon's surface can be seen from Earth. Causes : Lunar libration arises from three changes in perspective due to: 1. optical libration - its non-circular and inclined orbit. 2. parallax - the finite size of the Earth. 3. physical libration - the orientation of the Moon in space. Effects : The following are the four types of lunar libration: • Libration in longitude results from the eccentricity of the Moon's orbit around Earth; the Moon's rotation sometimes leads and sometimes lags its orbital position. The lunar libration in longitude was discovered by Johannes Hevelius in 1648. It can reach 7°54 ′ in amplitude. • Libration in latitude results from a slight inclination (about 6.7°) between the Moon's axis of rotation and the normal to the plane of its orbit around Earth. Its origin is analogous to how the seasons arise from Earth's revolution about the Sun. Galileo Galilei is sometimes credited with the discovery of the lunar libration in latitude in 1632, although Thomas Harriot or William Gilbert might have done so before. It can reach 6°50 ′ in amplitude. The 6.7º depends on the orbit inclination of 5.15º and the negative equatorial tilt of 1.54º. • Diurnal libration is a small daily oscillation due to Earth's rotation which carries an observer first to one side and then to the other side of the straight line joining the Earth's and the Moon's centres, allowing the observer to look first around one side of the Moon and then around the other - the observer is on Earth's surface, not at its centre. It reaches less than 1° in amplitude. Diurnal libration is one effect of parallax, which depends on both the longitude and latitude of the observer. [therefore is this true libration? - ed] • Physical libration is the oscillation of orientation in space about uniform rotation and precession. There are physical librations about all 3 axes. The sizes are roughly 100 seconds of arc. As seen from the Earth, this amounts to less than 1 second of arc. Forced physical librations can be predicted given the orbit and shape of the Moon. The periods of free physical librations can also be predicted but their amplitudes and phases cannot. Lunar and Solar eclipses: none predicted for this month Meteor showers: none predicted for this month For details regarding meteor watching, please see the SGAS 2021, pages 86- 87. 4 3. LOOKING UP SUGGESTED EVENING OBSERVATION WINDOW for AUGUST 2021 (Lunar observations notwithstanding) Date dusk end Moon 30th July 19h28 rises 23h57 (60%) 12th August 19h36 sets 20h09 (5%) CLUB STARGAZING – unfortunately, owing to the pandemic, we can still not enjoy physical club gatherings. Of course that should not prevent our members digging out a good coat and indulging in observation from your home or favourite darkest, rural, cloudless spots. And don’t forget the Moon, our closest celestial neighbour. Please consult our website for updates: http://www.hermanusastronomy.co.za DEEP SKY HIGHLIGHTS This month we revisit the “Big Five” of the southern African skies. These five targets are all visible to the naked eye while also richly rewarding the binocular or telescope user. What are the Big 5? The Big 5 are five celestial objects that represent the best specimens of each type of deep-sky class: • an open star cluster – the Southern Pleiades • a globular cluster - omega Centauri • a bright nebula - eta Carinae Nebula • a dark nebula - the Coal Sack • a galaxy - the Milky Way THE BIG FIVE of the southern African skies 6 Aug 2021 Omega Centauri Coalsack Southern eta Carinae NGC5139 Nebula C99 Pleiades IC2602 Nebula NGC3372 Constellation Centaurus Crux Carina Carina Distance 17 kly, 5.2 kpc 490 ly, 150 pc 479 ly, 147 pc 10 kly, 3.1 kpc Magnitude +3.7 +1.6 +1.0 Absolute mag -9.9 -4.43 -11.43 Apparent size 36 arcmin 375 x 250 arcmin 100 arcmin 120 arcmin Actual size 271 ly, 83.2 pc 53.4 ly, 16.4 pc 15.3 ly, 4.7 pc 349 ly, 107 pc Alt/Az 50º20’/ 125º10’ 51º45’ / 150º 40’ 59º21’ / 171º54’ 63º39’ / 168º54’ Declination − 47° 28.6′ − 63° 44.6′ − 64° 24.0′ − 59° 53.4′ J2000 RA 13h 26.8m 12h 31.3m, 10h 43.2m, 10h 44.3m, 5 Where can I see the big 5? The Big 5 are visible from anywhere within the southern hemisphere.
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