Observing the planets has been a hobby of humanity since the dawn of time. Our fellow travellers in the solar system have weaved their strange helical paths in the sky, forcing thinking and rethinking over what they are doing and why. Geocentric models of the Universe, centred on the Earth, required ‘epicycles’, extra little spins the planets did to explain why they sometimes moved one way and then another in the sky. Of course, we now know that the planets do this because as well as their own orbit, taking them to and throe along the Zodiac bouncing between points aligned either side of the Sun, the orbit of the Earth changes our perspective on the planets. When we undertake them, or are undertaken by them, in the great race around the Sun they appear to undergo retrograde motion – they reverse their direction in the sky, moving to the West rather than the East. Then it all goes back to normal. During this time, known as Opposition, they are at their closest to the Earth. The modern world has a total of eight planets, neatly cut into the inner Terrestrial Planets (including the Earth) and the outer Gas Giants, the two separated by the asteroid belt. Outside the outer planets lies the Kuiper belt of icy comets in waiting. For the purpose of observing the planets from the Earth, the term ‘inner planets’ is taken to mean the planets inside the orbit of the Earth. ‘Outer planets’ refers to planets which orbit at distances greater than that of the Earth, which includes Mars as well as the Gas Giants.
Planets appear star-like in the sky (always in the Zodiac, as this is where the ‘ecliptic’ plane lies where the planets and the Sun orbit) and identifying them, beyond being able to learn the stars of the constellations off by heart and noticing when an extra one pops in, is often a matter of looking at finder charts (perhaps in newspapers or local blogs). A more reliable way is to use a planetarium program like Stellarium.
The Inner Planets
The closest planet to the Sun is Mercury. Orbiting at some 58 million kilometres, observations of Mercury’s orbit played a crucial part in deciding that General Relativity and its predictions of how gravity worked was an improvement on the Classical Physics of Newton that went before. The planet itself resembles a shrivelled ball. It is a tiny planet, only around 5 thousand kilometres in diameter, and lacks a significant atmosphere. So insignificant does Mercury appear in the program of space exploration that the first full pictures of its surface have only been taken by probes orbiting the planet as I write. In many textbooks as a result, maps of Mercury have pieces missing.
This planet was first observed by the ancients. Its close orbit to the Sun means it follows the Sun very closely as it either sets of rises. By the time the Sun is far away enough set to be safe to look, Mercury is very close to the ground. This presents the obvious problem of how do you get a telescope to it before it has set as well as the less obvious problem of the warm air near the ground causing turbulence in the sky at dusk, and evaporating water causing much the same effect at dawn. If you do get to see this dot in the sky, you may be rewarded with the phases of Mercury, whose disk alters like that of the Moon. Of course, another time to see Mercury is when it transits the surface of the Sun, information on which can be seen in an earlier post.
Venus is the most obvious of the planets in the sky. A bright ball of light shining at either dawn or dusk, known as either the Morning or Evening star depending on where it is in its orbit, this planet might have caught your attention even when not looking for it. Venus is sometimes called Earth’s Twin, as at 12.1 thousand kilometres in diameter, it is closer to the Earth’s 12.7 thousand kilometres than any other planet. That is where the similarity stops. At 108 million kilometres from the Sun, Venus boils. Due to an atmosphere now composed mostly of CO2 and sulphurous compounds from the volcanoes on the surface, heat is trapped and heats up that surface above the temperature of the surface of Mercury. Venus is shrouded in clouds, with a surface only visible if a probe is fired underneath the clouds, where it will quickly melt and collapse on the surface. Recent observations have shown Venus to have tenuous aurorae over the entire dark side of the planet.
Observationally, Venus is very much like a more obvious version of Mercury. Being bigger, it reflects more light. Being further from the Sun, it appears in the hours of darkness, or is still present on the edge of darkness. Being an inner planet, it also shows phases and finally being shrouded in a single layer of cloud, it is generally featureless aside from these phases. It can also be seen transiting the Sun and has also been known since ancient times. Below is a picture of astronomers at the first event of the Eddington Astronomical Society in Kendal, which was formed at the meeting. It was a Venus watch with telescopes, binoculars, members of the public, a display on the planets and a feed showing the event as it was recorded from another observing site in case the haze defeated us (which in the end it didn’t).
The Outer Planets
All of the outer planets show a constant, nearly phase-less sunlit face to us, however, most of these faces are actually quite interesting to look at. Every one of them has at least a couple of satellites, which can be seen with the right power telescope, and the first three even have changeable features.
The first of these four is Mars, the Red Planet. Known for its ability to harbour martians in fiction and its strange red soil. Currently undergoing a program of exploration through the actions of the Mars Rovers as well as a series of Mars orbiting artificial satellites, our knowledge of the planet is as tenuous as it’s CO2 based atmosphere, and equally as interesting (I am an atmospheric physicist, I find tenuous atmospheres interesting, even CO2 ones). Mars lies 228 million kilometres from the Sun and is roughly 6.8 thousand kilometres in diameter. The planet has a slightly odd shape, perhaps due to a massive collision undergone at a time called the Heavy Bombardment phase, when small planetesimals slammed together to form the things we see today. Recent satellite observations
Observationally, Mars is yet another planet seen by the ancients. It does have two tiny moons, but these are difficult to spot. Things a little easier to spot are the massive regions of darker under soil that contrast with the brighter red regions of dust. Mars also has ice caps that can easily be seen to change with the seasons (with a moderately sized telescope). Small telescopes and binoculars should pic out the strange red disc and perhaps some surface markings, depending on what part of the planet is facing us at the time. Opposition, when Mars shines its brightest, will allow even small instruments the chance to see the markings clearly. Recent satellite observations have shown Mars to have small localised aurorae around magnetic hot spots.
Jupiter is known as the King of the Planets. A giant 143 thousand kilometres in diameter, 778 million kilometres from the Sun, this is a giant and a half. The planet contains observed storms that rage over distances greater than the diameter of the Earth. It is a Gas Giant, meaning unlike the rocky Terrestrial planets, the thing we actually see is a ball of gas. It isn’t known if the core formed into a solid thing inside or whether the pressure and temperature have formed something a little weirder down there, but it is known that the outer atmosphere is very active. The planet is also magnetically active. The first satellite to get close to Jupiter was fried by the radiation there, though it recovered enough to continue on its way with its remaining instruments. Jupiter was the first planet photographed to have a comet slam into it whilst the world watched. Fireballs larger than the Earth developed in the Jovian atmosphere that collapsed to form shock waves even larger in the atmosphere. The scars remained visible for a few years afterwards before the storms covered them up and the planet returned to life as usual. The auroral system on Jupiter, like everything else, is very much on a larger scale. Generated by particles expelled from the satellite Io, which end up in orbit of Jupiter and get ionised by the Sun, the Jovian auroral system puts out one hundred times the power of the Earth’s auroral system. It does have an Earth like aurora happening inside the main oval, but that is so weak in comparison to be almost irrelevant to the main one.
Observationally, this is yet another planet known to the ancients (they did a lot of planet gazing) and yet another visible in the night sky as a star-like object without the aid of telescopes etc. If you do look at it with binoculars or a small telescope, you are likely to see a yellow disc, perhaps with a bit of a red dot on it (the Great Red Spot) and maybe with two faint brown lines going across. These are part of the cloud band system, which can be seen more clearly with a larger telescope. Jupiter rotates in less than half a day, so it is possible to do widely spaced observations and see the red spot has moved across the disc. Jupiter also has four bright moons that can be seen in any telescope or binoculars. Galileo’s observation of these moons and recordings of where they were at each time, gave him the chance to work out how fast they went round the planet, which proved to be a good test of Kepler’s laws (indeed this is the basis for an experiment many astronomy courses run to familiarise people with Kepler’s laws).
Saturn is well known as the Ringed Planet. Presently under the undying eye of the Cassini Huygens probes, this planetary system has driven the imagination almost as much as Jupiter. A fellow gas giant, it lies 1.4 billion kilometres from the Sun and stands 125.1 thousand kilometres in diameter. It too has a strong magnetic field, but weaker than Jupiter. It has cloud bands, but they are fainter than Jupiter’s. It has an auroral system created through a combination of Earth like effects and ionisation of the rings, but these are less powerful than Jupiter’s, though they do save up their energy and produce massive bursts as the solar wind pulses rush over the planet.
Observationally, Saturn is the furthest of the eight planets to have been seen and recorded by the ancients. Through the smallest instruments, it can be seen to have rings (though, as it happens, they’re almost edge on at this moment in time, so they’re very hard to see for the first time in years). Saturn does have moons and yesterday four of them transited Saturn, due to the relative positions of the Sun, the Earth and Saturn, the shadows were visible on the planetary disc and even a small telescope would’ve seen the shadow of Titan. Sadly, this area was both clouded out and in the middle of the day at the time, so no show from here…
Uranus is the first of the outer planets not to be discovered in ancient times. Or at least not to be recorded as such, it does appear to the naked eye (under excellent dark skies and only just) at Opposition, but then goes very slightly below it, so the planet may have been seen and then simply vanished. It lies 2.9 billion kilometres from the Sun and follows the pattern of Saturn, in that it has very few bands at all, it is colder and smaller than Jupiter (just 51.2 thousand kilometres in diameter). It has very strange magnetic configurations. Whereas the strongest part of the Earth’s field is dipole (it has two poles, North and South), Uranus has strong quadrupole (four poles) and even octupole (eight poles) components to it’s field. The aurorae are weak and blobby (as the strange magnetic field makes nice smooth ovals difficult to produce). It was first discovered by Herschel in 1781, but it’s brightness allowed it to be mistaken for a star by many others including the astronomer Flamsteed, who saw it on six occasions (during the years 1690, 1712 and 1715), La Monnier recorded the planet six times in 1769 and Mayer recorded it once in 1756. Each time, it was recorded as a faint star, and when it moved, it simply got recorded as different faint stars. Such was the difficulty of observing these things.
Observationally, Uranus can only be seen using the naked eye in very good conditions, with excellent skies and the Earth close to the planet. Telescopically, the planet makes a lovely green disc among the stars.
Finally Neptune. At 4.8 million kilometres from the Sun and 48.6 thousand kilometres in size, this is the furthest and the smallest of the gas giants. Very little is known about it’s magnetic configuration and there is scant evidence of auroral activity at this moment in time. It was officially discovered in 1846 by John Couch Adams, but it has since transpired Galileo sketched it slightly earlier… He saw it twice in 1612 and even recognised that it was moving. However, there were no bright stars in the area to compare it to, so he may have assumed his memory was playing up.
Observationally, Neptune is like a smaller, fainter Uranus.