The Moon and Jupiter point the way to Uranus - September 2010

The planet Uranus lurks at the edge of naked eye visibility, but from anything other than the darkest, clearest skies it is effectively invisible without optical aid. Such good conditions are sadly lacking in the UK, and with Uranus fairly low in the sky anyway, having the big bright planet Jupiter nearby was a good opportunity to catch it.

Uranus lies at twice the distance of Saturn, the furthest planet known from before the invention of the telescope, and 5 times further than Jupiter. It takes 84 years to complete one orbit of the Sun, while Jupiter takes a mere 12 years, so every 14 years or so Jupiter overtakes Uranus - 2010 was one such year.

Here they are, on 23 - 24 Sept 2010, just a couple of days after their joint opposition from the Sun, and with the Moon for scale.

I confess that this picture is a bit of a cheat - it is of necessity a composite photo because the difference in brightness between the various celestial bodies would make it impossible to expose them all correctly on a single shot. Also, the Moon, 12 hours after full, was actually a few degrees away but I've brought it into shot to give a sense of scale (if only nature were really that obliging!). However, the positions of Uranus, Jupiter and of Jupiter's moons are genuine.

Roll your mouse over the picture to identify the objects you see...

Moon, Jupiter & Uranus on 23-24 Sept 2010

The Moon, Jupiter and Uranus captured on the night of 23 - 24 Sept 2010.

The nearby full Moon has been moved into the shot to give it scale, but in other respects the relative sizes and positions of the Moon and planets is genuine and corresponds to what you can expect to see through binoculars.

All in all, the picture gives a good impression of the view through binoculars or a small telescope. Also, the relative sizes of the Moon and Jupiter are genuine, but Jupiter's moons and Uranus are scarcely more than star like points in a small telescope so I over-exposed them to make them more obvious on the photo.

Their true sizes and distances are of course very different as the following statistics show:-
Earth: Diameter 12,756; mean orbital speed 29.8km/s
Moon: Diameter 3,476 km; light travel time 1.2 secs
Jupiter: Diameter: Polar 133,708 x Equatorial 142,984 km; light travel time 30 mins; mean orbital speed 13.1km/s
Uranus: Diameter: Polar 49,946 x Equatorial 51,118 km; light travel time 2 hr 40 mins; mean orbital speed 6.8km/s

You can also appreciate the distances involved if you consider that Jupiter's Galilean moons are comparable in size to our own - Ganymede is in fact slightly larger. Callisto appears the faintest here not because it's the smallest but because it is covered in a dark brown crust.

The greenish colour of Uranus is apparent if you look closely. Jupiter appears distinctly yellow in comparison to our grey Moon when seen visually, but is slighly over-exposed here so this is not obvious photographically.


Movement of Jupiter between 22 and 23 Sept 2010

The following shot was taken with the camera alone, no telescope involved. Jupiter is very overexposed, necessary to reveal the very much fainter Uranus, but roll your mouse over to see its movement between 22nd and 23rd Sept.

Jupiter & Uranus on 22 & 23 Sept 2010

Jupiter, its moons, and Uranus captured on a single shot. Use your mouse to see their movements with photos for 22 and 23 Sept aligned on top of each other.

Why does Jupiter move from left to right (east to west) when normally you expect planets (and the Sun and Moon) to move from west to east? Because we are just a day after Opposition, when the planets are opposite (180 degrees from) the Sun, and the Earth is overtaking them and leaving them behind. Only after nearly another three months, when the Earth has moved a quarter of its way around the Sun and will be heading directly away from them will normal forward motion be resumed. At this point Jupiter and Uranus will be approximately 90 degrees from the Sun, at a position known as Quadrature.

Of course these planets are really moving forwards all the time at the speeds mentioned above, but it's the parallax effect that causes their retrograde movement. Jupiter, being nearer, appears to moves backwards further and faster, while the movement of Uranus is barely perceptable night to night.

Once moving forwards, Jupiter will again catch up with Uranus and overtake it on 4 Jan 2011 with an even closer conjunction. They will be in the same binocular or low magnification telescope field of view for several days, and I'll post pictures if the skies are clear...


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