This view from NASA's Dawn spacecraft shows a portion of Mondamin Crater, an impact feature 78 miles kilometers in diameter, in the southern hemisphere of Ceres. Similar to many of the small, inner moons of Saturn, Prometheus points its long axis at Saturn as if giving us directions to the planet.
Pointing Toward Saturn. This will be one of Cassini's closest flybys yet of Titan, at just kilometers Titan Flyby - Sept. The tortured southern polar terrain of Enceladus appears strewn with great boulders of ice in this fantastic view, one of the highest resolution images obtained so far by Cassini of any world.
Boulder-strewn Surface -- Narrow-angle Camera View. This image of a complex pattern on the floor of Occator Crater was obtained by NASAs Dawn spacecraft on July 16, from an altitude of about 58 miles 93 kilometers. Complex Pattern on the Floor of Occator Crater.
Flowing Dunes of Shangri-La Denoised. These false-color mosaics from NASA's Cassini spacecraft chronicle the changing appearance of the huge storm that developed from a small spot in Saturn's northern hemisphere. Encircling a Giant. Titan's Tantalizing Streaks. Saturn's rings cast a dramatic shadow separating the blues and greens of the planet's northern hemisphere from the creamy pastels coloring the southern hemisphere.
This mosaic combines 6 images Shadow of Demarcation. NASA's Dawn spacecraft captured this tortured landscape just south of Ghanan Crater on Ceres on May 28, , at a distance of about miles kilometers above the surface. A crescent Enceladus appears with Saturn's rings in this Cassini spacecraft view of the moon. The famed jets of water ice emanating from the south polar region of the moon are faintly visible here Rings and Enceladus.
For example, if the Earth's distance from the Sun is one astronomical unit AU , then Venus's distance from the Sun is. However, no one knew the value of AU, so the absolute distances between the celestial spheres were not known. In , English astronomer Edmond Halley proposed a method for calculating our distance from the Sun—the astronomical unit—using the transit of Venus. The underlying principle behind Halley's method is something called parallax, the shift in position that comes from viewing an object from two different points.
What is parallax? Try this. Try this: Point your index finger towards an object on the opposite side of the room. Look at it first with just your left eye, then switch to looking at it with just your right eye. Can you see that the place across the room that your finger points to seems to jump, depending on which eye you use to look at it?
The farther away the object you point at, the more it seems to jump when you switch eyes. Observers stationed far apart on Earth are the two eyes, Venus is the pointing finger, and the Sun is the object across the room. Imagine two different people, one on each pole of the Earth, viewing the transit of Venus. The person on the North pole sees Venus following one path across the Sun. The person on the South pole sees Venus follow a slightly higher path, one that's shifted a little to the north.
Because we see the Sun as a circle, these two different paths will have different lengths. Halley proposed that an easy way to measure the difference between the lengths of these two paths would be to time the transits, using the four phases of the transit—the first, second, third, and fourth contacts—as indicators. With the two different paths known, the distance between the Earth and the Sun can be pretty easily calculated using trigonometry and Kepler's third law of planetary motion.
For over six hours, the planet Venus steadily inched its way over the surface of the Sun. For centuries, transits of Venus have drawn explorers and astronomers alike to the four corners of the globe.
And you can put it all down to the extraordinary polymath Edmond Halley. In November , Halley observed a transit of the innermost planet, Mercury, from the desolate island of St Helena in the South Pacific.
Calculating this angle would allow astronomers to measure what was then the ultimate goal: the distance of the Earth from the Sun. Halley was aware that the AU was one of the most fundamental of all astronomical measurements. Johannes Kepler, in the early 17 th century, had shown that the distances of the planets from the Sun governed their orbital speeds, which were easily measurable.
But no-one had found a way to calculate accurate distances to the planets from the Earth. The goal was to measure the AU; then, knowing the orbital speeds of all the other planets round the Sun, the scale of the Solar System would fall into place.
However, Halley realised that Mercury was so far away that its parallax angle would be very difficult to determine. As Venus was closer to the Earth, its parallax angle would be larger, and Halley worked out that by using Venus it would be possible to measure the Suns distance to 1 part in But there was a problem: transits of Venus, unlike those of Mercury, are rare, occurring in pairs roughly eight years apart every hundred or so years.
The person who deserves most sympathy is the French astronomer Guillaume Le Gentil. He was thwarted by the fact that the British were besieging his observation site at Pondicherry in India. Undaunted, he remained south of the equator, keeping himself busy by studying the islands of Mauritius and Madagascar before setting off to observe the next transit in the Philippines.
Ironically after travelling nearly 50, kilometres, his view was clouded out at the last moment, a very dispiriting experience. This is due to diffraction of light. While this showed astronomers that Venus was surrounded by a thick layer of gases refracting sunlight around it, both effects made it impossible to obtain accurate timings. But astronomers laboured hard to analyse the results of these expeditions to observe Venus transits. Johann Franz Encke, Director of the Berlin Observatory, finally determined a value for the AU based on all these parallax measurements: ,, km.
The AU is a cosmic measuring rod, and the basis of how we scale the Universe today. The parallax principle can be extended to measure the distances to the stars. If we look at a star in January - when Earth is at one point in its orbit - it will seem to be in a different position from where it appears six months later.
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