TONIGHT'S SKY - August 2021

Moon phases and distances

New Moon   Sun 8

First Quarter   Mon 16

Full Moon  Sun 22

Third Quarter   Mon 30

This month's Moon apogee (furthest from Earth) is on Mon 2nd at 404,410 km and perigee (closest to Earth) is on Tue 17th at 369,124 km.

Planets

Mercury has moved behind the sun and so not visible this month.

Venus is bright in the north-west before setting around 8.30pm. It is the second brightest object in the night sky (after the moon), our closest sunward neighbour, and thickly shrouded in cloud that reflects sunlight. It's both the 'morning' and 'evening' star. Explore why in a 2020 virtual planetarium show from Rochester Museum and Science Center that uses the free programs Stellarium (in a northern hemisphere view) and NASA's Eyes on the Solar System.

Mars is not visible this month and is about to pass behind the sun for some time. The Red Planet will be lost to our view for the rest of the year before returning in the east in the early morning skies next January.

Jupiter, the third brightest object in the night sky, is easily seen this month from around 7pm as it rises in the east, moves across to the west and fades by 7am in the early morning light.

Saturn, fainter and preceding Jupiter, the second largest planet can be seen rising in the east about 6pm before it too travels across the northern skies to set in the west around 6am.

Meteors

This month's major meteor shower is the Perseids which peaks on the 13-14th although not strong in the southern hemisphere. They are fast, bright, can leave persistent trails, and come from a point below the north-east horizon in the northern constellation of Perseus. These meteors result from Comet Swift-Tuttle which passed near the Sun in 1991 leaving a trail of particles for the Earth to regularly pass through.

Stars and constellations

In the north

This month's evening skies show Virgo and Spica (Alpha Virginis), the 15th brightest star at night and 262 light years away, have moved towards the west, while Leo has largely disappeared below the north-western horizon. However, Libra (the weighing scales) is high in the north and in the north-east is Aquila (the eagle) with its principal star Altair (Alpha Aquilae), the 12th brightest star at night and 17 light years from us.

In the east

Very high in the east after sunset and slowly moving overhead during the night as our planet rotates to the east is Scorpius with its impressive curving line of stars. The central star of the three that form the scorpion's body is the red giant Antares (Alpha Scorpii) at 34 light years and 16th brightest star at night. Rising behind and following during the night is the centaur Sagittarius with its bow and arrow forming the famous Teapot asterism.

In the south

Standing high in the south-west is Crux, or the Southern Cross. On a moonless night, and certainly away from city lights, can be seen the dark patch known as the Coal Sack nebula, a vast region of interstellar dust that blocks our view of more distant stars. To its left are the Pointers (Alpha and Beta Centauri), the brightest and second brightest stars in the constellation of Centaurus which is the other horse-human hybrid from ancient Greek myth.

Alpha Centauri is also known as Rigel Kentaurus ('foot of the centaur') and is the 4th brightest star in the night sky. Until very recently it was thought to be our sun's nearest neighbour at 4.37 light years. Although we see one star it is in fact two, Alpha Centauri A and Alpha Centauri B in close elliptical orbits around their barycentre or common centre of mass. The pair are not unlike our sun - one very similar and the other a little cooler and smaller. The existence of two planets, one around each star, is suspected but as yet unconfirmed.

Of special interest is the faint small red-dwarf star Proxima Centauri (designated Alpha Centauri C) that lies a considerable distance away but seemingly bound to the others. We now understand Centauri to be a triple star system.

Proxima, as the name suggests, is currently the closest star to our sun at 4.24 light years and hosts at least two planets. Proxima Centauri b (discovered in 2016) is a little over Earth mass, orbits swiftly much closer than Mercury does to our sun, and is presumed to be tidally locked with only one face turned to its star. The second planet Proxima Centauri c (discovered in 2016) is about seven Earth masses, lies further out in a 5-year orbit, and may possess a huge ring system.

Returning to our August night skies...

Low in the south-east is the 10th brightest star Achernar (in the constellation of the Eridani, the river) which sits at 144 light years from Earth, and in the south-west lies the 2nd brightest star Canopus in the constellation of Carina (the keel) at 313 light years. From our southern latitude both of these stars never disappear below the horizon.

These two stars provide are a handy example of a key relationship in astronomy and observing the night sky. A star's apparent brightness depends not only on its distance from us, but also its luminosity (how much light it emits). Luminosity is determined by size (radius) and surface temperature both of which can change over the life of a star as it undergoes stages of stellar evolution. Canopus is almost two and a half times further from us than Achernar but its luminosity is well over three times higher. On a league table of brightness as observed from Earth the energy output of Canopus easily wins over Achernar's closeness to us.

In the west

Low in the north-west is Arcturus, the 3rd brightest star at night and 37 light years from us in the constellation of Bootes (the herder or plower).

Corvus (Latin for crow) sits squarely in the west in the evening this month. In ancient Greek mythology, however, this sacred bird is a raven that perches on the back of Hydra (the sea serpent). The serpent's nearby head is a little higher up in the west and marks one end of a long narrow body that snakes to the horizon in the longest of the traditional 88 constellations.

This set of constellations was agreed upon by the International Astronomical Union (IAU) almost a century ago and drew heavily on European, Mediterranean or Middle Eastern traditions, sources and catalogues of the past. However, in this century new or supplementary names for celestial objects, as well as their stories, are increasingly being introduced from many cultures around the world to complement those already used widely in astronomy.

On this day

3rd 2004, the MESSENGER (USA) mission to Mercury was launched.

4th 2007, Phoenix (USA) Mars lander was launched.

5th 1998, NASA Near Earth Object Program is created to detect and catalogue asteroids that approach Earth.

5th 1939, first person to walk on the moon, American Neil Armstrong, is born.

6th 2012, the Mars rover Curiosity lands on the red planet.

6th 1996, a meteorite from Mars discovered in Antarctica is said by NASA to contain possible microfossils of bacteria.

7th 1959, Discoverer 1 (USA) returns the first satellite images of the Earth.

10th 1675, Royal Greenwich Observatory is established east of London.

10th 1990, Magellan (USA) arrives at Venus and begins radar mapping of its surface.

12th 1877, Astronomer Asaph Hall at the US Naval Observatory discovers Mars' tracks are clearly eviden.t 12.6km diameter moon Deimos.

13th 1898, Eros, the first near-Earth asteroid is found by Carl Gustav Witt.

17th 1970, Soviet probe Venera 7 is launched to Venus and will send first pictures from the surface of another planet after landing on December 15th.

18th 1877, Mars' 22.5km diameter moon Phobos is discovered by Asaph Hall.

18th 1868, new element ‘helium' is found by Pierre Janssen from analysis of the Sun's spectrum. It is now known to be the second most abundant element in the universe after hydrogen.

19th 1960, two dogs, Belka and Strelka, are launched aboard Sputnik 5 (USSR), and successfully returned to Earth.

19th 1646, birth first Astronomer Royal John Flamsteed who catalogued 3000 starts.

20th 1975, launch of Viking 1 (USA), first probe to land on and study Mars.

20th 1977, Voyager 2 (USA) launched to the planets of the outer Solar System.

22nd 1989, Voyager 2 (USA) discovers positive evidence for Neptune's rings.

24th 2006, first formal definition of 'planet' is debated and vote upon by International Astronomical Union in Prague resulting in dwarf planet status for Pluto.

25th 1609, Galileo demonstrates to the Venetian Doge and officials his improved version of the newly invented telescope.

31st 1913, birth of famous British radio-astronomer Bernard Lovell.