How to see the planets for yourself

You have almost certainly heard much about the planets of our Solar System, and have likely seen the beautiful pictures space probes have made of each of them from nearby. But what some people don't know is that you don't need to go to other planets to see them: they're all visible from Earth. In fact, a couple of them are just about the easiest things to find in the sky, and each of them is fun to see. So for those who are curious, but haven't seen the planets yet (or not all of them) I have written a little guide, starting with the easiest planet to see in 2013 and working my way up to the hardest. Give it a try some night, seeing another planet is not much harder than seeing the Moon once you know where (and when) to look! All you need is at least one eye and a clear sky.

Jupiter

(Image credit: NASA)

The Romans named Jupiter after the king of their gods, and if you've ever seen it on a dark night, it's not hard to see why. Jupiter is far brighter than any star and easily dominates the night sky if the Moon isn't out. This makes it the easiest planet to see in many years, including 2013. Jupiter is very bright due to its enormous size, which means it reflects a lot of light.

Right now, Earth is passing between Jupiter and the Sun, and this means Jupiter is at its closest to us(and therefore brightest), and that Jupiter rises right as the Sun sets and sets as the Sun rises. That makes any cloudless night these months an awesome time to look at Jupiter. In the early evening, you can see it shine brightly in the east, even from the most light-polluted cities. Around midnight, it passes very high through the south, easily the brightest “star” in the sky. Early in the morning, it sinks down into the west as the Sun rises in the east. Due to Jupiter's brightness, you probably won't need a star chart or any knowledge of the sky to find it. It's brightness makes it stand out easily: for most of the night it's the brightest object in the sky other than the Moon (which, by the way, will approach Jupiter very closely in the sky on 26 December, and even cover it seen from South-America and Africa). But in case you need more help, it's in the constellation of Taurus. It's easy to find due to a large hexagon of very bright stars in the sky, often called the Winter Hexagon. It's near the star marking its right corner, Aldebaran, but way brighter than this star:

(Image of the brightest stars of the winter sky made using SpaceEngine. Jupiter is the bright object on the right of the large hexagon. Brighter stars are larger in the picture than smaller ones. It helps to look at this picture in full view)

While Earth is passing between Jupiter and the Sun now, it will slowly move away during 2013's early months. Jupiter will stay in the same area of sky, but will set earlier every day. By March, it'll be in the south around sunset, and sink into the west during evening, setting around midnight. In May and June, Jupiter will pass on the other side of the Sun and therefore be invisible from Earth. But in July it will become visible again very early in the morning, rising about three hours before the Sun in the east. In the final months of the year, Earth will approach Jupiter again and it will rise earlier all the time, so by December 2013 it will once again be visible the whole night long.

(Image credit: NASA)

Jupiter is a beautiful sight with the naked eye, but if you have binoculars or a telescope you can see something very cool: its four largest moons! They're called Io, Europa, Ganymede, and Callisto, and each is the size of a small planet (Ganymede is in fact larger than Mercury). The only reason we can't see them with the naked eye in the first place is that Jupiter's bright light is in the way, but with binoculars, even poor ones, you can see them separate from Jupiter. They're visible as four “stars” close to Jupiter (or three, or two, or occassionally just one; they often pass behind or before Jupiter so they're not visible), roughly on a straight line. If you look at them from day to day, you can see they're in different places each time due to their fast orbits. With good binoculars it also becomes possible to see Jupiter as more than a point of light, but actually see it as a sphere. You won't be able to see its beautiful clouds, though: you need a telescope for that.

Venus

(Image credit: NASA)

Maybe you have seen something very strange in the sky once. A mysterious light hovering above the western horizon in the evening, or maybe it was above the eastern horizon early in the morning before the Sun was up. It was obviously far too bright to be a star, but unlike a plane it didn't move at all. It just hung there silently. Was is a spaceship from another world?

Well, no. In fact, it was another world: Venus. Venus is ludicroudly bright, as much as six times brighter than Jupiter. Other than the Moon and the Sun, nothing in our skies is brighter. She gets so bright because A: she's the closest planet to Earth, B: she's covered in bright white clouds, reflecting a lot of light, and C: she's close to the Sun, so brightly lit. Venus is bright enough even to cast shadows in places that are very dark. (in any place in or near a city these very faint shadows are washed out by the light pollution though)

So if Venus is so bright, why do I consider Jupiter the easiest planet to see? Well, there's a catch: Venus is closer to the Sun than Earth, so in the sky she's always near the Sun. Jupiter can be nearly overhead in the sky in the middle of the night, but Venus is always near the Sun so it's usually day when she's in the sky. However, at dawn or dusk she is often visible and makes even Jupiter look faint then. Right now Venus rises shortly before the Sun, but she's about to pass behind it, becoming invisible to us Earthlings from mid-January on. She won't emerge on the Sun's other side until June, and then she'll be visible low in the evening sky for the rest of the year. She'll follow the setting Sun, setting one to three hours after it and gracing the western evening sky with her brightness. You really don't need a star chart to see Venus; it's a matter of looking to the west in the second half of the year about an hour after the Sun sets: Venus is unmistakable.

Much like Jupiter, Venus also has a bonus if you look at her with binoculars. They need to be binoculars on a tripod, though: holding them in your hand won't make them steady enough. But if you have binoculars on a tripod and aim them at Venus, you can probably just make out that the planet doesn't look round, but has the shape of a crescent Moon. This is because Venus is closer to the Sun than Earth, so we're looking at it more or less from behind.

I've said it before: Venus is bright. But in fact, she's so bright she's visible at day! You've probably seen the Moon at day some time, but not Venus. This is because Venus is very easy to miss. Unlike the Moon, she's just a tiny little dot of white among a sea of blue sky. But she's visible if you know where to look. October will likely be the best time to try: Venus will be at her farthest away from the Sun. To try it, find a spot where the Sun is just out of your sight: this will prevent your eyes hurting from looking roughly in its direction. So you need to be in the shadow, but left of the Sun (in the east) you need to be able to see the sky. Sitting in a place where a tree or building covers the Sun is probably the best idea.

Venus will be a bit over forty degrees to the Sun's east. To measure out these forty degrees, spread your hand out at armlength. Provided you are not very weirdly proportioned, the distance from the tip of your thumb to the tip of your pinky is roughly twenty degrees at arm's length. So you look two of these distances left of the Sun. The best times to do this would likely be 14:30 (at that time, Venus will approximately at the same height as the Sun). Most likely, however, you will see nothing there at first, just blue sky. Well, keep looking at the area, scanning it carefully with your eyes: the detail of a tiny white spot against the blue sky is too slight to be picked up by your peripheral vision, so you need to see it with the centre of your sight. Don't give up: it takes a while to find.

There's another situation when Venus is visible at day: when it passes exactly between the Sun and Earth. It's visible as a little black dot during these Venus transits. This happened in June, but unfortunately won't happen again until 2117.

Saturn

(Image credit: NASA)

Just like Jupiter, Saturn is huge. But it's almost twice as far away from the Sun, so it gets less light, and is also further away from Earth. Because of this Saturn isn't as bright as Jupiter, but it's still a bright planet. It's about as bright as the brightest stars, so you'll need a little knowledge of the constellations to recognise it. Not much, though, as it's bright enough to easily stand out among all but the brightest stars. Saturn is in between the constellations Virgo and Libra at the moment, and fortunately there's only one bright star nearby:

(Image of the spring sky near Saturn made using SpaceEngine. The circled object is Saturn. Looking at the picture in full view helps)

Saturn doesn't rise until the morning right now, not long before sunrise. But in the first few months of 2013 it'll rise earlier all the time, until Earth passes between Saturn and the Sun in April, when it'll be visible all night long as it rises when the Sun sets and sets when the Sun rises. From June to September it'll set during the night, and therefore will be visible in the evening only. During October and November, Saturn will pass behind the Sun, and therefore will be invisible. In December it'll start rising before the Sun in the early morning again.

If you look at the southern sky in April or May, you will see two bright “stars” near each other, with a third one higher up in the sky. Saturn is the brightest of these three, at the lower left of the triangle they form. It's also the only one that's yellow-whitish; Spica (the star near Saturn) is a little bluish, while Arcturus (the star above Saturn and Spica) is a bit reddish. There are likely other stars visible as well, but none of them are as bright as Saturn.

With a binocular on a tripod, Saturn's beautiful rings can just be seen. To see them clearly separate from the planet you need a telescope, but in binoculars the planet looks oddly elongated, a bit egg-like due to the rings. Saturn also has one big moon, Titan, which is just in the range of binoculars. It's very big, bigger than Mercury, but Saturn's distance to the Sun makes it a lot harder to see than Jupiter's big moons. A faint star nearby Saturn is likely to be Titan, particularly one that moves from day to day.

Mars

(Image credit: NASA)

After Venus, Mars is the closest planet to Earth. This is both a blessing and a curse for seeing it: when Earth passes between Mars and the Sun, the planets get very close and Mars becomes brighter than any star, sometimes even brighter than Jupiter. It's a fiercely bright red thing in the sky at these times. But Mars' proximity also means it moves very fast compared to Earth, and that at its furthest, it's as much as five times farther away than its closest. It's very faint by comparison during these times, and this is one problem.

The other problem is this: Mars orbits the Sun once every 687 days*, while Jupiter takes twelve years over an orbit. This means that if Mars and Jupiter were both behind the Sun seen from Earth (and thus invisible), we need to wait only half a year for the Earth to pass between Jupiter and the Sun: Jupiter moves little in that time. Mars, on the other hand, will be long gone after half a year, and has completed a good part of its orbit by then, so while it's not precisely on the opposite side of the Sun any more it's still quite far away and it'll be another half year before Earth gets close to it.

This means there are great Mars years, when the planets are close to each other all year long and Mars is really bright. It also means there are awful Mars years, when Mars is in hiding behind the Sun most of the year. 2013 is one of these awful years, and this is why I've listed it behind Saturn (in a good year I might have listed it as the easiest to see planet). In January 2013, Mars sets less than two hours after the Sun. After this, it'll pass behind the Sun for most of the year, and won't be visible again until August, when it'll start rising before the Sun in the early mornings (along with Jupiter, but Jupiter will be as fiercely bright as always while Mars will look like an average star). It'll slowly rise earlier and become brighter, but even in December it still doesn't rise until 1:00 and is no brighter than Saturn.

Mars' rapid movement also makes it hard to give a star chart for its location, as it moves constellations about once every two months, and is constantly on the move within those constellations. So instead of making a dozen star charts, I will tell you in which constellations it is in each month. Then you can look that constellation up on Wikipedia for its star chart; the bright star that's not on it will be Mars. In December 2012 and January 2013, it's in Capricorn, low in the southwest in the evening. In July, it reappears in the northeastern morning sky in Taurus. In August it'll be in Gemini, and in September in Cancer. It will spend October and November in Leo, and end the year in Virgo.

Mercury

(Image credit: NASA)

Mercury is the smallest planet, but it's also closest to the Sun and therefore very brightly lit. This makes it quite bright despite its size: under favourable conditions it's brighter than any star, though not as bright as Jupiter. There's one problem, however: it's closer to the Sun than even Venus. While Venus gets a decent distance away from the Sun, Mercury is always sticking very close to it and never visible out of twilight. Even when it's visible in twilight, it'll be low above the still-lit horizon the Sun just set under/is about to rise above. This makes it far more difficult to see than the previous planets. In fact, I have never seen it myself.

When Mercury gets far enough from the Sun to be visible in twilight, it's only for a few weeks at most: the planet orbits in only 88 days, so it moves fast. It's visible in the early mornings right now, but only until 20 December. It's pretty close to the far brighter Venus, actually. After 20 December, it'll pass behind the Sun, but it'll be visible in the evening, just after sunset, from 8 to 26 February. Then it'll pass between Earth and the Sun, but when it reaches the other side of the Sun it'll never come high enough to be visible. But half an orbit later it'll reemerge in the evening sky from 19 May to 22 June, getting quite close to Venus once again in June. It passes before the Sun, and shows up in the mornings again from 24 July to 16 August. When it reaches the other side of its orbit it'll once again never get high enough to see, and its final visibility of the year will be from 7 November to 7 December.

I've established how hard Mercury is to see, but there are things that make it easier: Venus or the Moon being close by is the main one, since they are bright enough to be seen easily in twilight. The Moon (a crescent) will be close to Mercury on 11 February, 10 and 11 July, 5 August, and 1 December. From 24 to 27 May it'll be close to Venus and Jupiter. It'll also pass near Saturn on 26 November, but since Saturn is fainter than Mercury this is unlikely to help much. A star chart wouldn't be of much help either, as the Sun's glow near the horizon washes out the stars.

Uranus

(Image credit: NASA)

Uranus is only half as big as Saturn, and twice as far away from both Earth and the Sun's light. Due to this, it's far dimmer, so dim in fact it wasn't discovered until 1781. Uranus is just bright enough to be seen with the naked eye outside of cities, but this is hard to do. With binoculars or a telescope it's visible quite easily, with just one problem: how do you distinguish it from thousands of dim stars?

Well, with a map of course. I've found it myself using a map like the one linked below: it's a tiny blue light in a telescope, which becomes a minuscule little blue sphere at high magnification. Right now Uranus is in Pisces, where it'll be visible in the evenings during the first months of the year, behind the Sun in April and May, visible in the morning from June to September, and visible all night long as the Earth passes between it and the Sun in October and November. It's a difficult planet to find, but very cool to have seen.

The chart is located here: http://dcford.org.uk/findercharts.php?obj=uranus&year=2013. It's no mistake that Uranus doubles back on its path in August: this is a perspective effect caused by Earth overtaking it.

Neptune

(Image credit: NASA)

Neptune is even farther away than Uranus, and so even dimmer. It's theoretically visible to the naked eye under absolutely ideal conditions (very, very far away from any lights (We're talking Antarctica here), an incredibly clear sky, preferably on a mountain top or in space, when the Sun is very far beneath the horizon and Neptune high in the sky, there's no Moon, no Venus, and you've got very good eyes. Then you might just be able to glimpse a hint of it), but I don't think anyone has ever actually seen it with the naked eye. It's still visible through binoculars and telescopes though (though I have never seen it myself). You'll need a map of the area of Aquarius where it can be found (best in August, when Earth passes between Neptune and the Sun). This map is fortunately supplied by the same site that gave us the one for Uranus: http://dcford.org.uk/findercharts.php?obj=neptune&year=2013

*=Coincidentally** exactly as long as an elephant pregnancy.

**=Or not. Maybe elephants come from Mars.

Curiosity and the exploration of Mars

As you've probably heard by now, the Curiosity rover landed on Mars yesterday morning (or the night of the day before yesterday, if you live in the Americas). This is the next big step in the exploration of Mars that has been going on for millennia. “Millennia?” You may ask. Yes, millennia, because the exploration of Mars didn't simply start with the first spacecraft sent there, but much before that. With its discovery, to be precise.

It's unknown who first discovered Mars, but what is known is that he or she lived thousands of years ago. The ancient Egyptians, Babylonians, and Chinese all knew of the existence of Mars and the other four planets that can be seen with the naked eye a thousand years before Rome was founded. You see, as they observed the stars, they realised all stars moved a little each night, rising and setting four minutes earlier than the night before. But always by that amount; it was as if the sky was a giant sphere with light on it that rotated around the Earth very slowly. But seven things in the sky didn't obey the rules: there were the Sun and the Moon, but aside from them there were five bright stars that moved a little among the stars each night. The Romans called them Mercury, Venus, Mars, Jupiter, and Saturn, after several important gods.

It was thought that these five, the Sun, and the Moon, all moved in circles around the Earth below the stars. The Earth was in the centre of the universe, then you had the Moon, then Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and beyond Saturn were the stars. There were some oddities that were difficult to explain like this, however. Mercury and Venus always stayed close to the Sun, for example, never reaching the opposite side of the sky. Mars, Jupiter, and Saturn did reach the opposite side of the sky, where they would be said to be in opposition, but once there they did a very strange loop in the sky over the course of a few weeks.

Mars was the most troublesome of the planets for this geocentric model: not only was the loop it made at opposition biggest of all three planets that did one, but it did many other things that perplexed ancient astronomers: it would stray off the path the other planets followed across the sky a little, and it would vary in brightness. One year, it would be brighter than anything in the sky except for the Sun, the Moon, and Venus. The next, it wouldn't be all that impressive, and all the other planets would get brighter, as would a couple of stars. It would also vary in brightness a lot during a year: all planets were brighter at opposition than when they were close to the Sun, but Mars showed much bigger variation than any of the others.

The big loop at opposition and the difference in brightness over the year were explained in 1543 when Nicolaus Copernicus published a book in which he claimed the Earth wasn't at the centre of the universe, but the Sun. The Earth was the third planet moving in a circle around it, and Mars the fourth. This would bring them very close together at opposition, much closer than Earth and Jupiter, for example, so by comparison they would be much further away from each other when Mars was on the other side of the Sun. The loops Mars, Jupiter, and Saturn made in the sky were a perspective effect caused by Earth passing those planets, and Mars made the biggest because it was closest.

However, Mars' deviation from the path of the other planets and its differing brightness across the years wasn't explained until 1615, when Johannes Kepler realised the planets didn't move in circles around the Sun, but in ellipses. Sometimes they got closer to the Sun, and sometimes further away. Most planets have an orbit very similar to a circle, but Mars' orbit is more elliptic than all others (except Mercury), getting about ten percent closer to the Sun at its nearest (which is called perihelion) than at its farthest (which is called aphelion). The difference in brightness was because in some years, Mars reached opposition while at its closest the Sun (and therefore the Earth), while in others it was at its furthest from the Sun.

During the 17th century, as telescopes were being pointed at the sky for the first time, Mars was found to look bigger at times when it was brighter and smaller when it was dimmer. This made perfect sense, of course, since the planet was sometimes closer to the Earth than at other times. The first details on Mars were being seen in this century and the next as well: white polar caps and dark spots on its reddish surface. It was found to rotate in 24 hours and 39 minutes, just a bit slower than Earth, and it had an atmosphere. Mars seemed a world that was a lot like Earth: the dark spots could be oceans, or perhaps forests.

In the 19th century, this view was confirmed even more as Giovanni Schiaparelli saw long, straight lines on Mars. Some thought these were canals built by the Martians to irrigate the dry areas of the planet. At the time, canals were a huge new product of technology, the Suez canal having just been completed while work had begun on the Panama canal, so canals were a big part of public consciousness. There were even ideas to help the Martians see us as we saw them, by bigging trenches thousands of kilometres long in the Sahara, filling them with oil, and lighting them on fire! A Mars-craze erupted, as it was considered very likely there was life on Mars by now. H. G. Wells wrote a great book, The War of the Worlds, about a Martian invasion of Earth, which was among the first science-fiction books.

However, around 1900 telescopes became better, and it was getting clear the dark spots on Mars were no seas, as they showed mountains and canyons in them. The canals turned out to be an optical illusion caused by smaller spots and craters lying roughly in lines. The concept of Martians remained a big part of culture and speculation for much of the 20th century, but for a few decades Venus was considered a more likely planet to harbour life by science. When the first interplanetary probes were launched in the 1960s, they were therefore going to Venus, but when they discovered that planet to be far more hellish than expected, Mars once again gained the spotlight. It was highly unlikely by now that little green men lived there, but there could be plant life or microbes on the red planet.

The Russians were the first to send probes to Mars. The first six, from 1960 to 1964, all failed; four of them before even reaching Earth orbit, and the other two while on their way to Mars. The Americans built two probes to Mars; Mariner 3 and 4, but Mariner 3 failed as well. It was almost getting suspicious, seven probes all failing before they got to Mars. Maybe the Martians existed after all and were protecting their secrets? Fortunately Mariner 4 launched perfectly, three weeks after Mariner 3's failure. It became the first successful probe to fly by Mars, and for the first time showed photographs taken of another planet from close by, showing a rough landscape full of craters and mountains. It also showed Mars to be harsher than expected: the atmosphere was very thin, providing only 1/150th of Earth's pressure. This also means the planet's heat easily escapes into space at night, bringing temperatures down below a hundred degrees below zero, though it also warms back up quickly when in daylight, sometimes reaching temperatures similar to a summer's day on Earth: 27 degrees. For the most part, its daytime temperatures stay quite a bit under freezing, though.

It made multicellular Martian life unlikely, as very few Earth organisms could survive long in conditions like that. But a subsequent set of probes launched in 1969, and managed to get into orbit of Mars, observing it for a while. They found structures that looked like dried-up riverbeds, and discovered the tallest mountain in the entire Solar System on Mars. It's called Olympus Mons, and is nearly 22 km high. With the discovery that water may have once flowed on Mars, it was clear the only way to find out more about the planet was to land on it.

In 1975, this was finally accomplished by the Viking 1 and Viking 2 landers, which each landed on the surface after making orbital photographs for a while. They each carried three experiments intended to detect microbial life if it existed on Mars. Two of them turned out negative, but a third was positive. The most common explanation of this is that life wasn't detected, but the third experiment set off by other factors, however, this is far from certain. For the most part, the Viking experiments proved inconclusive. Yet attention turned away from Mars, and this coupled with big budget cuts to both the American and Russian space programs meant Mars was mostly ignored for twenty years.

By the 1990s, it became possible not just to land on Mars, but to ride around there, so experiments could be done on many more places than just the single location the Vikings had been able to. The first such rover, Sojourner, was a small, cheap thing with few instruments on board, mainly intended to prove the rover concept was doable. In the next few years, three probes were sent to Mars: the American Mars Global Surveyor and Mars Odyssey, and the first European mission to Mars, called Mars Express. These three probes researched and photographed Mars in greater detail than ever before, and found many traces of water on Mars: dried-up riverbeds and lakes, gullies, and even signs of ancient rainfall. Global Surveyor failed in 2006, but the other two are still in orbit and returning data every day.

Along with Mars Express, the European Space Agency had tried to put a rover named Beagle 2 on Mars, intended to find signs of life, but contact was lost with the rover during landing, and it has presumably crashed or otherwise failed. The Beagle 2 was intended to land in a rather strange way: by surrounding it with inflated balloons and simply letting it bounce on Mars' surface until it would finally stop, at which point the ballons would be deflated and the rover released. The next year, however, NASA landed two rovers, Spirit and Opportunity, on Mars using the same method. These two mainly researched the geological history of Mars, including the period, long ago, in which there was liquid water there. Opportunity has done so for eight years by now, but Spirit became stuck in soft soil in 2009 and was eventually terminated the next year after many unsuccessful attempts to get it loose.

And now, finally, we get to Curiosity. Like always happens in science, the answers Spirit and Opportunity provided led to even more questions. And so the largest and most advanced rover yet was launched. Curiosity was lowered from a floating platform by a crane yesterday (no, I'm not making this up, that's really how it was landed. The balloon method was simply too dangerous to use with the car-sized Curiosity), and returned its first pictures not much later. It's intended to work for at least two Earth years (one Martian year), but will most likely continue to work for several years afterwards. One of the things it's going to start looking for is the building blocks of life. Its data will also serve to prepare a future manned Mars landing. Curiosity landed in the Gale crater, where there is a mountain called Aeolis Mons, which it'll be slowly climbing over the course of its mission, while examining many of the things it will come across.

What happens next? Well, it partially depends on Curiosity's findings. It is likely Curiosity will be followed up with an even more advanced rover around 2020, which might finally settle the question of life on Mars. And by the 2030s, a manned mission will land on Mars, most likely after earlier manned missions to an asteroid and to Mars' moon Phobos. Several countries have plans for Mars missions: America and Russia, of course, but Europe, Japan, China and India as well. The most likely way for a manned mission to Mars to happen, however, is as an international mission. The International Space Station has proved big, expensive space missions are best performed through international cooperation, sharing both the expenses and the knowledge and expertise required. A manned mission to Mars will likely require such international cooperation to succeed. Once on Mars, there is the possibility they would stay for several months and conduct research rovers can't do; but there is also a different idea floating around: it could be a one-way trip. The astronauts could be sent with everything required to set up the first Mars base and stay on Mars, greatly reducing both the cost and difficulty of the mission, while immediately beginning the colonisation of Mars.

Well, that turned out a little longer than expected. I hope you enjoyed reading it, though, or otherwise stopped reading it when you realised you didn't enjoy it.