Sundials!

Above is an analemmatic sundial. The idea is to orient the sundial facing south, and then place a vertical pointer on the central figure-8 track, in a position corresponding to the date. The sundial above shows a simulated shadow for 2:15 PM yesterday. It can be seen that the sundial tells the time reasonably well, thanks to the inbuilt adjustment for variation in solar position.

For large-scale analemmatic sundials, like the one below, people can stand on the central figure-8 track and act as a human pointer. A sundial like this is fun to have in the garden.

Here are blank sundials for some Southern Hemisphere cities:

The ShadowsPro software will also generate sundials like these, if anyone is particularly enthusiastic.


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Mars at Closest Approach


Image from NASA (Australians must look East)

On Tuesday 31 July (early evening in Australia, early morning in the US, 07:45 GMT), Mars makes its closest approach to the earth since 2003. The actual distance is provided by Wolfram Alpha.


Image from fourmilab


The constellation Scorpius

Winter is here (in the Southern Hemisphere, at least), and the constellation Scorpius always heralds the southern winter’s icy sting. The image below is based on a vintage astronomical illustration, but I have corrected the star positions of the major stars and indicated their apparent magnitude (brightness) and approximate colour (based on spectral class). It is interesting to compare the image with this quality photograph.

Generations of astronomers have memorised the O–B–A–F–G–K–M stellar classification system developed by Annie Jump Cannon with the mnemonic “Oh, Be A Fine Girl/Guy/Gal/Gentleman, Kiss Me.” Scorpius does not contain any bright O-class stars, but it is easy to see stars ranging from the hot blue-white B class to the cooler orange-red M class (stars which are only “red hot”).

The most obvious star in Scorpius is the enormous red supergiant Antares, which has that name because it is easily confused with the planet Mars (Ares). It is also known as “Cor Scorpii” (the heart of the scorpion). It is easy to recognise the curved tail as well, with the stingers Shaula and Lesath at its tip. It is less obvious which stars are the scorpion’s claws – the artist here has drawn the left claw extended so as to reach the dim white star Psi Scorpii. Other artists draw the scorpion facing more to the right, with the line of blue-white stars being the claws.

Infographic constructed using R (with lm to map true sky coordinates to image coordinates, rasterImage for the background, and the showtext package for fonts).


Her pale fire she snatches from the sun

Shakespeare writes “the moon’s an arrant thief, and her pale fire she snatches from the sun” (Timon of Athens, Act 4, Scene 3). He is, of course correct. The moon merely reflects sunlight, and produces no light of its own. One way of telling this is that moonlight actually displays the same telltale absorption spectrum as sunlight:

Our eyes tend to perceive moonlight as “blueish” or “silvery,” but that is because of the way our eyes work at low light levels. Long-exposure photographs under moonlight, like this one, look much like daytime shots:

Anaxagoras (499–428 BC) seems to have been the first to discover that the moon shines only by reflected light:

Anaxagoras also explained that solar eclipses occur when the moon moves between the earth and the sun. Total solar eclipses are dark precisely because the moon produces no light of its own:


Travelling across Australia: Ten things to spot

On October 8, teams in the World Solar Challenge begin their race from Darwin to Adelaide. Here are 10 things for travellers across Australia to look out for.

1. The Magellanic Clouds

The Magellanic Clouds are two small galaxies – at 160,000 light-years and 200,000 light-years, the nearest visible galactic neighbours of our Milky Way. They can be seen in the Southern Hemisphere, away from towns. The Australian Outback is the perfect place to observe them.


The Magellanic Clouds (photo: ESO/S. Brunier)

2. The Southern Cross

The Southern Cross (Crux) is a constellation appearing on the flags of many countries in the Southern Hemisphere, including Australia. It consists of four bright stars, with a fifth being visible to the naked eye in good conditions. The constellation can be located with the aid of the pointer stars Alpha Centauri and Beta Centauri. It can also be used to determine the South Celestial Pole. The star at the “top” of the Cross (Gamma Crucis) is a red giant. The fifth star (Epsilon Crucis) is an orange giant.


The Southern Cross, pointers, and Magellanic Clouds (image: Michael Millthorn)

3. The wedge-tailed eagle

The wedge-tailed eagle (Aquila audax)) is Australia’s largest bird of prey, and a national icon. It can be seen around Australia, either in the sky, or snacking on roadkill.


Wedge-tailed eagle (photo: Fir0002/Flagstaffotos)

4. The red kangaroo

The red kangaroo (Macropus rufus) is the largest living marsupial, and is found throughout central Australia, in areas with less than 500 mm rainfall. It is an Australian national icon, as well as being a major traffic hazard at dawn and dusk.


Red kangaroos (photo: Jenny Smits)

5. The sand goanna

The sand goanna (Varanus gouldii) is a large monitor lizard, growing to about 1.5 metres. It is found across much of Australia.


Sand goanna (photo: Alan Couch)

6. The thorny devil

The thorny devil (Moloch horridus) is found in arid, sandy areas of western and central Australia. It lives mostly on ants.


The thorny devil (photo: Bäras)

7. Magnetic termites

Magnetic termites (Amitermes meridionalis) are one of two Australian termite species building mounds that align north–south. They can be found in the vicinity of Darwin. The mound orientation appears to be a temperature-control mechanism.


A magnetic termite mound (photo: brewbooks)

8. Sturt’s desert pea

Sturt’s desert pea (Swainsona formosa) grows in arid regions of Australia. It is the floral emblem of the state of South Australia.


Sturt’s desert pea (photo: Fir0002/Flagstaffotos)

9. The desert grasstree

The desert grasstree (Xanthorrhoea thorntonii) is a grasstree found in arid regions of western and central Australia. Like the other 27 species of grasstree (Xanthorrhoea spp.), it is endemic to Australia, and a symbol of the Australian landscape.


The desert grasstree (photo: Mark Marathon)

10. Opal

Opal is a gemstone form of hydrated silicon dioxide. The town of Coober Pedy in South Australia is a major source.


Opal from Coober Pedy (photo: Dpulitzer)


Silly Season in the Sky (again)

It’s apparently time for lunatic end-of-the-world prophecies again. The latest relates to a “great red dragon” in the sky (a reference to Revelation 12):

Turns out that this is a double image of Saturn, taken way back in 1983 by the Infrared Astronomical Satellite (IRAS). Saturn was then at RA 13h45m18.2s, DEC -08d13m07s.

It’s a false-colour image (i.e. not red at all), being taken at 100 microns, in the infrared region of the spectrum. But with enough spin, apparently it can be made to sound scary.

In the final version of the infrared sky survey, this artefact was blacked out, since it doesn’t reflect any actual stellar infrared sources (just a planet that moves around). Of course, that removal got the conspiracy-theory nutters going.