My imagination is that of an engineer with astronomy post-grad qualifications. What I see in the more typical image below is more akin to a caricature the classical illustration of a Dodo (Head to the right, feet to the bottom). In Steve’s image I see only exquisite detail including Bok Globules (top centre) and lanes of silhouetted dust.

Image credits:
S. Crouch, 2008. IC 2948, Running Chicken Nebula
M.Lorenzi and G.Favretto, 2003. IC2948 Complex

Needless to say, the image is not a simple happy snap of the planet and its moon. The NASA site carries a description of how the image was composed but it also carries quite a bit of jargon that I’ll try to decode here.
The camera responsible for the image, called LEISA, is sensitive to infrared light: the sort of light emitted by your TV remote. Light is characterised by its wavelength, with infrared light having a slightly longer wavelength than the nearby visible red light. The warmer an object is, the shorter the wavelength of the bulk of light it emits… if it is warm enough it glows visibly red, yellow, blue… think stars. While Jupiter’s atmosphere is not hot enough to emit much infrared light just because of its temperature it does reflect a lot of sunlight. Different gases reflect/absorb light at particular wavelengths and this can give away the atmospheric composition. By taking images in light of a range of wavelengths (1.59, 1.94, 1.85 micrometres) the camera is effectively taking images of gas of certain compositions, temperatures and, by inference, depths in the atmosphere. Using comparative information such as these images a great deal can be learnt about the object. The three images above show an approximate reproduction of the original images around the Great Red Spot (GRS). The imaging team shaded the images red, green, and blue and merged them to form the image of Jupiter.

The overlaid image of Io was constructed using a monochrome (black and white) optical photograph taken with the Long-Range Reconnaissance Imager (LORRI) camera. Using images from another camera, sensitive to bluish light typical of methane, an artificial colouring has been applied. Once more Tvashtar is billowing, and lava is flowing on the surface.

You can experiment with splitting light with a spectroscope by following the links from my previous Science Projects in a Small World post. The LEISA camera is tuned to pick up light from one narrow section of a full spectrum, similar to targeting the prominent lines in the image to the left.

• The Wigglez of the Universe from the Centre for Astrophysics and Supercomputing
• Towards Absolute Zero on an Atom Chip from the Centre for Atom Optics and Ultrafast Spectroscopy (CAOUS)

Go here for viewing pleasure.

On a slightly different tack is the creatively off-beat Make Magazine. Make publishes a traditional dead-tree magazine that rarely makes an appearance on Australian shelves (although I think McGills carry it) and a regular podcast program. Ever wanted to make your own Rubik’s cube out of dice, Jam Jar Jet! (PDFCast), or Teeny Tiny Solar Robots.

Enjoy.

You might recall the hubbub surrounding last year’s International Astronomical Union (IAU) decision to put a definition on the term planet. The fact that Eris was initially assessed as larger than Pluto (2390 km), and that there were likely to be many more objects like it, partly drove the need to define planet. Pluto, the last planet to be discovered, was relegated to the new category of dwarf planet to join the asteroid Ceres and Eris.

The ways that astronomers have found to gather information about things they cannot even hope to get close to never ceases to impress me.

Clear Skies!

See also:

[1] Brown, M. E.; Trujillo, C. A.; Rabinowitz, D. L. Discovery of a Planetary-sized Object in the Scattered Kuiper Belt. The Astrophysical Journal, Volume 635, Issue 1, pp. L97-L100. NASA ADS Entry

[2] Bertoldi, F.; Altenhoff, W.; Weiss, A.; Menten, K. M.; Thum, C. The trans-neptunian object UB313 is larger than Pluto. Nature, Volume 439, Issue 7076, pp. 563-564 (2006). NASA ADS Entry

[3] Brown, M. E.; Schaller, E. L.; Roe, H. G.; Rabinowitz, D. L.; Trujillo, C. A. Direct Measurement of the Size of 2003 UB313 from the Hubble Space Telescope. The Astrophysical Journal, Volume 643, Issue 1, pp. L61-L63. NASA ADS Entry

[4] Michael E. Brown and Emily L. Schaller. The Mass of Dwarf Planet Eris. Science, Vol. 316. no. 5831, p. 1585. Journal link

Written in glorious Fortran 77, SWIFT could be built with many commercial Fortran compilers including Intel’s Fortran Compiler. I successfully built SWIFT using the Intel compiler although I never used it in anger. I was never able to build it with the GCC 3.x Fortran (g77) because of the code’s use of recursive routines. Two things have changed in this regard:

• GCC 4.1+ now includes gfortran, which aims to be Fortran 95 compliant.
• David E. Kaufmann has authored a completely redesigned version of the Swift package with the support of NASA’s Applied Information Systems Research Program (AISRP).

The enhanced package is called Swifter and is written to Fortran 90 standards. It now compiles with the GCC gfortran compiler with the following change made to the Makefile.Defines:
FORTRAN = gfortran
FFLAGS = -O -ffree-line-length-none

Now I can attack the problem of building funky animations.

Clear skies!

Clear Skies!

Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Tvashtar in Motion

• Eyeballs
• A set of binoculars with 50mm or larger objective (front) lenses; the common variety at this size are labeled 7×50 (7 refers to magnification and 50 the lens diameter in millimetres).
• A clear sky to the east around 8 PM (harder to arrange than the binoculars).
• Some idea where to look.

Read on for help with the last item (at least for Brisbane, Australia)

Finding Jupiter in the sky tonight could not be easier. Face east at about 8 PM after giving your eyes about ten minutes in the dark to adapt. Look about 40 degrees above the horizon for the brightest ’star’ in the area. You can use the map to help orient yourself (click for larger version). The reasonably easily identified constellation of Scorpius lies slightly higher in the sky. Train your binoculars on that ’star’.

With a modicum of luck you will see a small bright disc with a line of small dots around it. These dots are some or all of the Galilean Moons: Io, Europa, Ganymede, and Callisto. The top diagram (right) gives you some idea of what to expect in binoculars. With binoculars you will not see colour or the bands on the planet’s surface, and you might not see the innermost moon (Io) which is easily lost in the glare of the planet. The bottom diagram is a telescope view with moons labeled.

What Galileo noticed is that the dots moved while he watched, but they never moved too far from the planet. By watching the planet over a period of a couple of hours you will see the dots moving. Io takes a just 42 hours to orbit Jupiter once, so it is feasible to see movement over a single night’s viewing. Europa takes 85 hours, so movement is quite easily seen from one day to the next. If you are particularly observant you may notice that the planet has moved against the background stars from one night to the next.

Galileo made a series of drawings of the planet and its moons over time. From this information he was able to derive the orbital period of the moons and demonstrate that they orbited Jupiter and not the Sun. The orbits of both Jupiter and its moons are very well known now. Astronomers can now plot in advance exactly where to expect he moons in relation to Jupiter. A typical presentation of this information is shown above for five days either side of 6 June. The vertical centre line represents Jupiter, and the four coloured, waving lines give the position of the moons. Time is read of the left axis.

I hope this guided observation session has whet your appetite as it did mine the first time I gazed upon the largest of our celestial companions. If you want a better look your local astronomy club can offer guidance and often rental telescopes.

Clear skies!

The finder chart and other static views were created using Stellarium. Stellarium is available for windows, Mac OS X, and Linux. The Jupiter moons animation was Created using XEphem, post-processed with ImageMagick and Mplayer. The plot of future moon positions was extracted from KStars.

The unenhanced image can be seen to the right (click for larger version). In this image we can clearly see ring (annulus) of light around the planet’s edge caused by sunlight being refracted (bent) through the outermost atmospheric layers toward the camera. The ring system, Saturn’s most distinctive feature, is brightly lit and clearly visible beyond the extent of the planet. Where the rings pass in front of the planet they appear dark, obscuring the weak illumination of the planet’s dark side by light reflected from the rings. The broad, dark band in the rings is visible from Earth in modest amateur telescopes and is called the Cassini Division after its discoverer Giovanni Cassini. The Cassini division was thought to be a vacant gap between the A (outer) and B (inner) rings. Just inward from the very bright edge of the ring system (F ring, discovered 1979) is the Encke Gap named for Johann Encke. Near the planet’s lower left edge is a bright spike that hints at another ring, and below the planet is a faint blue streak that one could be forgiven for thinking was some kind of lens flare.

The image truly comes alive when the brightness levels are adjusted (left). The Cassini Division no longer looks like an abrupt break in the rings as more, faint, rings appear in the ‘gap’; their locations dictated by orbital resonance with Saturnian moons. The bright spike has resolved into a ring (G ring) discovered by the Voyager mission in 1980-1. The pale blue streak is fully illuminated, betraying its nature as a full ring (the E ring, discovered 1966). The character of the reflected light tells us that E ring differs from the majority of the rings; it consists of diffuse microscopic particles rather than larger lumps of ice and rock. The source of this material is thought to be volcanism on the Saturnian moon Enceladus which orbits within the haze. New rings, not easily visible in this scaled image were discovered by Cassini scientists in this image data (“Janus/Epimetheus” Ring between F and G, and “Pallene” Ring betwee G and E).

Spectacular as the image of Saturn is, the thing that really tugs on my sense of wonder is an almost insignificant dot called the Earth that makes an appearance just inside the G ring in the centre-right. At the time this image was taken Cassini-Huygens was about 2.2 million kilometres from Saturn and 1.5 billion kilometres from Earth. A similar image, in which the Moon can be seen as a bump on the side of Earth is here.

Read more about the Saturn ring system at Wikipedia.

Image Credit: NASA/JPL/Space Science Institute
PIA08329: In Saturn’s Shadow
PIA08324: Pale Blue Orb

Clear skies!

The outer circle of the image on the tee is marked in French with the twelve months of the year. Inside that ring is the inscription ‘Orbite ou route de la terre autour du soliel’ meaning something like ‘Orbit or road of the earth around the Sun’ (Soleil is the modern spelling). The other marking ‘Sytème de Ptolemée’ indicates that the diagram is of the Ptolemaic System (or geocentric system). The four circular symbols at March, June, September, and December depict the Earth, its equator, tropics and polar circles, and its orientation to the sun at this time.

The device in the middle is an armillary sphere (a term that escaped me on the night). The armillary does indeed have an astronomical purpose in that it is an abstract representation of the sky and the paths of objects in it, particularly the Sun. The armillary was developed by the Greeks around the third century BCE, and the Chinese around the first century BCE although simpler devices existed prior to this. The device can be used to make astronomical measurements of star positions, as the ancient Chinese, Greeks and Ptolemy did, or as a navigational aid. Many armillary spheres have a purely instructional purpose, demonstrating the relationship between various points, planes, and paths in identification of points in the sky. Armillaries are fairly common as purely ornamental objects.

In early armillary spheres the object at the centre represented the Earth because everybody knew the Earth was the centre of the Universe. Some later spheres depict the Sun at the centre after the Copernican view. The Earth is supported by a rod that forms the north-south axis of rotation and points to the north and south celestial poles. Surrounding the central object are a series of rings of various sizes and orientations and sometimes an object representing the Moon. The exact numbers of rings vary from device to device although the major ones are always present.

The rings can be divided into two groups; the great circles and others. There are typically five great circle rings:

• The projection of the Earth’s equator to form the celestial equator (Label A). This ring lies on a plane perpendicular to the axis through the Earth and is often marked with twenty-four divisions representing the hours of a day and used to identify the right ascension of an object. Right ascension is analogous to longitude on the Earth’s surface.
• The meridian ring (Label L), mounted on the top and bottom of the axis rod and crossing the equatorial ring at right angles. This ring is typically marked in degrees from zero at the equator through to 90 at the poles and is used to indicate the declination of a point in space. Declination is the astronomical equivalent of latitude on the Earth’s surface.
• The apparent path of the Sun through the sky over a year is represented by a ring (Label B) mounted at an angle of approximately 23 degrees to the equatorial ring. The angle comes from the tilt the Earth’s axis has. The ring is marked, often ornately, with twelve divisions representing the zodiacal constellations and its width may represent the range in which the Moon and classical planets wander in (23 plus/minus 9 degrees). This ring lies in a plane called the ecliptic plane. Aries is aligned with the intersection of the equatorial ring where this ring passes from the southern to northern hemisphere.
• The equinoctial colure (Label G) passes through the two poles and the intersection of the equatorial and ecliptic rings. Equinoctial means “equal night”, is related to the term equinox, and identifies the location of the Sun on two days when day and night are of equal length (roughly 20 March and 23 September).
• The solsticial colure (Label H) passes through the two poles and is perpendicular to the equinoctial colure. Its intersection with the ecliptic ring identifies the location of the Sun at its northern- or southernmost point in the sky during the year. These times are called the solstices and are approximately 21 June and 22 December. These are the longest and shortest daylight periods of the year.

Often a disc is present to represent the local horizon of the observer (Label M). The sphere can be mounted on a movable stand (as in the diagram) to allow it to be tilted and rotated for use at various latitudes and times.

Smaller rings are mounted to represent:

• The antarctic and arctic circles. These rings are parallel to the equator and approximately 23 degrees from the poles.
• The Tropic of Cancer approximately 23 degrees north of the equator and coincident with the northernmost excursion of the ecliptic. The name comes from the position of the Sun in the constellation Cancer at the June solstice… at least it was when the name was given. The Sun is now in Taurus at the June solstice because of the effect of precession.
• The Tropic of Capricorn approximately 23 degrees south of the equator and coincident with the southernmost excursion of the ecliptic. The naming rationale is similar to that for the Tropic of Cancer except for the December solstice (now Saggitarius).

The armillary sphere also makes a stylised appearance on the Portugese flag. Officially it symbolises the seafaring prowess of the Portuguese throughout history and particularly during the reign of King Manuel I (1495–1521) when Portugal was a major maritime leader. Astronomical observation and instruments like the armillary has a long history in navigation, particularly at sea where there are no landmarks. The sphere also features prominently in Portuguese Manueline architecture.

Want to build yourself an armillary sphere? Try here.

Clear skies!

We all know that stars twinkle, their brightness varies a little as the atmosphere disturbs our view. There are, however, occasions when a star will completely disappear and reappear a few seconds later. These events are called occultations although a related term might be more familiar; eclipse.

Astronomical occultations occur when an object passes in front of of another completely obscuring it from view. For example, a total solar eclipse is the occultation of the Sun by our Moon. The occulting body might be the Moon, a planet or one of its moons, or an asteroid or other minor body. In the case of our Moon, it is happy coincidence that its size almost perfectly matches the Sun’s size when viewed from their respective distances. This allows the 3,474 kilometre diameter Moon to completely blot out the 1,392,000 kilometre diameter Sun’s glare allowing the faint corona to shine through.

Far less obvious occultations occur from time to time when an asteroid in the solar system’s asteroid belt passes between us and the light from a distant star. Stars are tiny points of light in the night sky, so we cannot expect to see things like the corona and other effects we see during solar eclipses. The best we can hope for is that the star will temporarily disappear from view, or significantly dim, while the asteroid passes in front. On 20 April this occurred when asteroid 324 Bamberga occulted HIP 59807. Prior to the event, predictions had been made using the known, fixed (almost, but that’s another post) position of the star and the known orbit of the asteroid. These calculations allow plotting of the predicted path of the ’shadow’ over the surface of the Earth with known degree of confidence.

Amateur astronomers like these events because it allows them to measure the asteroid’s size and get an idea of its shape. If you measure the time that the star is blotted out you can calculate the diameter of the asteroid. By spreading out across the expected path of the occultation each observer gets a slightly different line of sight and slice through the asteroid’s body. Combining the measurements allows a shape to be derived. For this event five Canberra area observers saw the star disappear, and two in New Zealand did not see the event. Dave Gault’s video of the event can be seen on YouTube. Assuming a roughly elliptical shape, this occultation gave 216.2 by 229.4 km as the best-fit dimensions for Bamberga. Even failing to see the star disappear is useful; if the observer’s point of view passes just outside the body of the asteroid. Such observations can be used to better refine orbital information and put an upper bound on the object dimension.

Occultations are one area where amateurs can add useful data to the scientific pool. Science you can be involved in is always of interest to me.

Other Bits

The HIP 59807 designation comes from the catalogue constructed by the Hipparcos mission. The two plots used here were shamelessly lifted from:

• Steve Preston’s asteroid occultation pages
• Royal Astronomical Society of New Zealand Occultation Section Bamberga/HIP 59807 result page