Australians know that the world is round

Following up on my earth-measuring post, people have known for more than 2,000 years that the earth is round. In 350 BC, Aristotle wrote “The evidence of the senses further corroborates this [that the earth is spherical]. How else would eclipses of the moon show segments shaped as we see them? As it is, the shapes which the moon itself each month shows are of every kind straight, gibbous, and concave-but in eclipses the outline is always curved: and, since it is the interposition of the earth that makes the eclipse, the form of this line will be caused by the form of the earth’s surface, which is therefore spherical. Again, our observations of the stars make it evident, not only that the earth is circular, but also that it is a circle of no great size. For quite a small change of position to south or north causes a manifest alteration of the horizon. There is much change, I mean, in the stars which are overhead, and the stars seen are different, as one moves northward or southward. Indeed there are some stars seen in Egypt and in the neighbourhood of Cyprus which are not seen in the northerly regions; and stars, which in the north are never beyond the range of observation, in those regions rise and set. All of which goes to show not only that the earth is circular in shape, but also that it is a sphere of no great size: for otherwise the effect of so slight a change of place would not be quickly apparent.” (On the Heavens, II, 14).

Around the year 700, Bede wrote “We call the earth a globe, not as if the shape of a sphere were expressed in the diversity of plains and mountains, but because, if all things (terrestrial) are included in the outline, the earth’s circumference will represent the figure of a perfect globe. Hence it is that the stars of the northern hemisphere appear to us, but never those of the southern; while on the other hand, the people who live on the southern part of the earth cannot see our stars, because the globe obstructs their view.” (De Natura Rerum). Australians verify his statement about stars every night.

I have commented previously on how the medieval poet Dante described time zones on a round earth:

In more recent times, we have pictures from space:

Aristotle and Bede mention the stars. Not only do the visible stars vary with latitude, but in the Northern Hemisphere they rotate around Polaris, while in the Southern Hemisphere they rotate around the South Celestial Pole, as in this photograph taken in Chile:

Sailors at sea have long known that the earth is round. From a vantage point 20 metres above sea level, one can see a complete ship 17 km away. Beyond that, the distant ship goes “hull down,” and only the upper parts of it are visible (from 34 km away, the lower 20 metres of a distant ship will be hidden). Closer to sea level, the distance is much less. This photo, taken in Spain by “Santifc,” shows the phenomenon (and similar observations can be made at some Australian beaches):

And, of course, the aircraft flight times to and from Australia can only be explained by the fact that the earth is round:


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The moons of Saturn

In honour of the Cassini probe, the diagram above shows (to scale) the larger moons of Saturn (those over 100 km in diameter). The moons are shown in order of distance from Saturn, with Janus and Epimetheus being the closest. Titan comes between Rhea and Hyperion.

Janus, Epimetheus, Mimas, Rhea, and Phoebe are shown in black-and-white. Enceladus, Tethys, and Dione are shown in enhanced colour (combining infrared, visible, and ultraviolet). Titan is shown in infrared, which penetrates the atmospheric haze. Hyperion is in true colour, and Iapetus in false colour. Click on the image to zoom.


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.


Spaceprobes in the Solar System

The diagram above (click to zoom) and list below show currently active spacecraft in the Solar System, not including those operating close to the Earth (and I’ve probably missed a few):

  • Voyager 1 and Voyager 2, launched in 1977, now heading into the darkness and still reporting back, even though they are over 17 billion km or 16 light-hours away (not shown above).
  • Cassini–Huygens, launched in 1997, now in the final stages of its exploration of Saturn (see fact sheet).
  • 2001 Mars Odyssey, launched in 2001, and still orbiting Mars.
  • New Horizons, launched in 2006, now en route from Pluto to the Kuiper belt object 2014 MU69, which it should reach in January 2019.
  • Dawn, launched in 2007, currently orbiting Ceres (image shown above the Earth).
  • Akatsuki, launched in 2010, currently orbiting Venus, which it will do until 2018.
  • Juno, launched in 2011, currently orbiting Jupiter, which it will do until February 2018.
  • Hayabusa 2, launched in 2014, currently en route to asteroid 162173 Ryugu, which it should reach in 2018. It will then take a sample which should arrive back home in 2020 (not shown above).
  • ExoMars Trace Gas Orbiter, launched in 2016, currently orbiting Mars and mapping the Martian atmosphere (the associated Mars lander was lost in 2016).
  • OSIRIS-REx, launched in 2016, currently en route to asteroid 101955 Bennu, which it should reach in 2018. It will then take a sample which should arrive back home in 2023 (image shown below the Earth).

In addition to the above, BepiColombo is scheduled to launch for Mercury in October 2018, and SolO is scheduled to launch for the Sun that same month. Also, InSight is scheduled to launch for Mars in May 2018, and Solar Probe Plus is scheduled to launch for the Sun in August 2018.


Metallic hydrogen

It has long been known that hydrogen forms a metallic solid at moderate to low temperatures and extremely high pressures. Last year, Ranga Dias and Isaac F. Silvera at Harvard University were the first to actually produce this metal, using the enormous pressure of 495 gigapascals (see their paper).

At higher temperatures, hydrogen forms a metallic liquid, and this is believed to exist at the heart of Jupiter and Saturn. This liquid has yet to be observed.


The ExoMars lander

The ESA/RFSA ExoMars lander seems to have crashed on Mars, unfortunately. The chart above shows the planned sequence of events (which was to have ended with a soft collision). Instead, we seem to have had an early parachute release, with a very brief use of thrusters, followed by loss of signal after 19 seconds – consistent with free-fall to the surface under Martian gravity.

Update: the crash site seems to have been identified.