Curiosity · Chapter 12 — Beyond Earth

In the Nubra valley in Ladakh, the twins Yangdol and Dorjay love the night sky. The air there is clear and dry, there are almost no clouds and very few lights, so on a dark night the sky is filled with thousands of stars. Their elders tell them how, long ago, travellers in camel caravans crossing Nubra used certain stars to find their way.

Stars shine with their own light. Some look bright and some dim. Looking at them, people have always tried to “join the dots”. Try it: look at a picture of a part of the night sky, connect some stars with imaginary lines, and see what shape you get: a fish, a kite, a cot, a hunter? Give your pattern a name and make up a story about it. Compare with friends: everyone’s patterns and stories will be different, and that is part of the fun.

Ancient star patterns. Our ancestors did exactly this. Watching the sky was a favourite pastime long before electric lights. Different cultures saw different shapes in the same stars and named them after animals, objects and characters in their stories. These imaginary shapes made it easier to recognise and remember the stars.

Why were star patterns useful?

• Finding direction. Before modern technology, even before the magnetic compass, sailors at sea and travellers on land found their way by recognising stars. This skill is still used as a backup in emergencies.
• Keeping time. Different patterns are seen at different times of the night and in different seasons, so they helped people know the season.
• Stories and culture. Star patterns carry the stories of many communities.

An important caution. The lines joining stars are not real; they are drawn only to help us recognise the pattern. And the stars in one pattern only appear close together from the Earth. In reality, some may be much farther away than others.

Why is the sky so clear in Ladakh? High, dry places with few clouds and very little light from towns let us see many more stars. For this reason, India has built an observatory at Hanle in Ladakh, and the area around it was declared the Hanle Dark Sky Reserve in December 2022.

What is a constellation? In earlier times, a group of stars forming a pattern was called a constellation. Different cultures drew different boundaries. So, in the early twentieth century, the International Astronomical Union (IAU) agreed on one set: the whole sky was divided into 88 regions, and these regions are now officially the constellations. Each region includes a familiar star pattern, and because the pattern is the most noticeable part, people still use the word constellation for the group of stars too.

Orion, the Hunter. Orion is one of the easiest constellations to find. Look for three bright stars in a short straight line in the middle; they are imagined as the belt of a hunter. Around them are bright stars marking his shoulders and knees. One story imagines Orion with his dog (the constellation Canis Major) facing a bull (the constellation Taurus).

Sirius. Canis Major contains Sirius, the brightest star in the night sky. To find it, follow the line of Orion’s three belt stars towards the east (in the direction away from Taurus); it leads to dazzling Sirius.

When to look. In India, Orion is best seen after sunset between December and April.

Indian names. Indian astronomy uses the word nakshatra for a particular star or a group of stars.

Stars are suns. Each of these stars is a huge, hot ball of gas like our Sun, and some are much bigger than the Sun. They look like tiny points only because they are extremely far away.

Try to make your own sky art. Some people embroider constellations on dark cloth or prick them on black paper and hold it up to a lamp. Making them helps you remember them when you look up at night.

Two of the best-known star patterns in the northern sky look like ladles or dippers: the Big Dipper and the Little Dipper. Each has four stars forming a “cup” and three stars forming a “handle”.

The Pole Star appears nearly stationary. As the night goes on, other stars seem to move across the sky, but the Pole Star stays almost in the same place, in the north. So if you can find it, you know which way is north. This is how travellers in the Northern Hemisphere found direction. The Pole Star is not very bright, and it cannot be seen at all from the Southern Hemisphere.

Finding the Pole Star with the Big Dipper.

• On a summer evening around 9 p.m., face north and find the Big Dipper.
• Look at the two stars at the outer edge of its cup, the end away from the handle. They are called the pointer stars.
• Imagine a straight line through these two stars, going out of the open top of the cup.
• Go along this line about five times the distance between the two pointer stars. You reach a star that is not very bright: that is the Pole Star.

Watch the Big Dipper move. If you look at the Big Dipper three or four times, two or three hours apart, you will see that it seems to turn around the Pole Star, while the Pole Star itself stays put.

Local stories. Many communities have their own stories. Some tribes in central India see four stars of the Big Dipper as a grandmother’s cot and the other three as thieves trying to steal it. Fishermen along the Konkan coast see the four stars as a boat and the other three as its neck.

If you live in a big city, you may see only a few stars, even on a clear night. Why? Because of light pollution, smoke and dust. Light pollution is too much artificial light at night: street lights, lit hoardings, floodlights and windows that shine upward and brighten the sky, hiding the fainter stars. Tall buildings and trees can also block your view. In villages and dark open areas, far more stars can be seen.

Light pollution is growing fast around the world. It not only hides the stars; it can disturb birds, insects and other animals that are active at night. To protect dark skies for research and for everyone’s enjoyment, dark sky reserves have been set up, like the one at Hanle in Ladakh, and some groups teach people how to reduce unnecessary lighting.

Not every star is seen from everywhere, every night. Different stars are visible in different seasons and from different parts of the Earth. To know when and where to look, you can use sky-map apps on a phone, such as Sky Map or Stellarium (Stellarium also has a free computer version).

Planning a night of sky watching.

• Always go with an adult. Never go to a dark open place alone at night.
• Choose a dark, open place away from lights, buildings and trees.
• Use an app to choose the date and time for what you want to see.
• To find faint stars such as the Pole Star, pick a night with no Moon and no clouds.
• Carry a sky map or printouts, a magnetic compass to find directions, and a notebook to draw what you see.
• On reaching the spot, wait about half an hour for your eyes to get used to the dark. You will then see many more stars.

Instruments. Binoculars and telescopes make objects in the sky look brighter and larger, and a telescope shows many faint objects that the naked eye cannot see. Planetariums, science museums, amateur astronomy clubs and colleges often hold night-sky watching events where you can look through one.

Doing your bit. Switch off lights you are not using, and use lamps that shine down, not up.

The Sun is a star, and it is the star closest to us. It is an extremely hot, ball-shaped mass of gases that gives out a huge amount of energy, which is why it glows so brightly. Its heat and light are the main source of energy on the Earth. Sunlight lets plants grow, which give food and oxygen to animals and people. The Sun also drives the seasons, the weather, the winds and the water cycle.

How big is the Sun? Its diameter is about 100 times that of the Earth. Yet in the sky it looks no bigger than a plate. That is because it is very far away.

How far? The Sun is about 150 million km from the Earth. Such distances are hard to imagine, so astronomers use a handy unit for distances within the Solar System, the astronomical unit (au). One au is approximately the distance between the Earth and the Sun.

Why does the Sun look so different from other stars? The Sun looks big and bright because it is much, much closer to us than any other star. Other stars are so far away that they look like tiny points, even though some are much bigger than the Sun. During the day, the Sun is so bright that we cannot see any other star; the stars are still there, but their faint light is lost in the glare.

The next nearest star. After the Sun, the nearest star is Proxima Centauri, about 269,000 au away. That means it is about 269,000 times as far from us as the Sun.

Safety. Never look directly at the Sun, not even with sunglasses, binoculars or a telescope. It can permanently damage your eyes.

The Sun in Indian culture. Being the brightest object and the source of light and heat, the Sun was given the status of a deity in many ancient civilisations. In India it is worshipped as Sūrya.

The Sun, the Earth and many other objects together form our Solar System. Most of these objects move around the Sun. The movement of an object around the Sun is called revolution.

What is a planet? A planet is a large, nearly spherical object that revolves around the Sun. The Earth is a planet: it takes nearly one year to complete one revolution. At the same time it rotates (spins) about its own axis, once in about 24 hours, which gives us a day. Other planets also rotate while revolving.

The eight planets. Here they are in order of increasing distance from the Sun:

Inner and outer planets. The four inner planets, Mercury, Venus, Earth and Mars, are smaller and have solid, rocky surfaces. The four outer planets, Jupiter, Saturn, Uranus and Neptune, are much larger than the Earth and are made mostly of gas and ice. These giant planets have rings made of dust and rocky material around them.

Hot and cold. Planets get most of their energy from the Sun, so in general the farther a planet is, the colder it is. But an atmosphere can trap heat. Venus has a thick atmosphere, so it is hotter than Mercury although it is farther from the Sun.

Venus, the “Morning Star”. After the Sun and the Moon, Venus is the brightest object in our sky. It is seen near the east before sunrise or near the west after sunset, so it is called the Morning Star or Evening Star, though it is not a star at all.

Pluto. Pluto lies beyond Neptune and is smaller than our Moon. It was once called a planet, but in 2006 the IAU decided that it and other similar small objects are dwarf planets.

Planet or star? Mercury, Venus, Mars, Jupiter and Saturn can be seen with the naked eye as bright dots. Stars twinkle a lot; planets usually do not twinkle much.

Objects that move around planets are called satellites. The moons of planets are natural satellites. They are smaller than the planets they go around.

In general, any object that moves around a much bigger object can be called a satellite; in that sense, the Earth is a satellite of the Sun.

Our Moon. The Earth’s natural satellite, the Moon, is our nearest neighbour in space, about 3,84,000 km away. It takes about 27 days to go once around the Earth. Its diameter is about a quarter of the Earth’s. There is hardly any atmosphere on the Moon.

Craters. The Moon’s surface is covered with round, bowl-shaped hollows called craters. Most were made when asteroids and rocks from space crashed into it. Because the Moon has no air, no water and no life, nothing wears these craters away, so they last for a very long time.

India and the Moon.

To mark the Chandrayaan-3 success, India celebrates 23 August as National Space Day.

How long would it take to get there? Distances in space are huge, but the Moon is close compared with the planets and stars. A car moving at 100 km every hour, without stopping, would need 3,84,000 ÷ 100 = 3840 hours, which is 160 days, to cover the distance. Spacecraft travel much faster and reach in days.

Remember that the Moon does not make its own light. Like the planets, it shines because it reflects sunlight.

Asteroids. The Sun and planets are nearly round, but the Solar System also has many small, rocky, irregular objects called asteroids. They range from about 10 m to about 500 km in size. Many revolve around the Sun between the paths of Mars and Jupiter, in a region called the asteroid belt. Now and then, an asteroid passes close to the Earth.

Comets. Sometimes we get visitors from the outer parts of the Solar System: comets, made of dust, gases, rocks and ice. As a comet comes close to the Sun, its frozen material starts to evaporate and forms a long, glowing tail. As it moves away from the Sun, it fades and cannot be seen with the naked eye. Many comets return to the Sun again and again. The famous Halley’s Comet appears about every 76 years; its last visit was in 1986, so it is expected back around 2061. In Sanskrit a comet is called Dhūmaketu; some tribes call it Pucchya-Taro (star with a tail). People once feared comets as signs of bad luck, but science shows they are just icy, rocky travellers.

The Solar System, in one line. The Sun, eight planets and their moons, and many smaller objects such as asteroids and comets together form the Solar System. The Sun is the largest and heaviest member and makes almost all its energy. All the other members shine only by reflecting sunlight.

The Milky Way. On a moonless night, from a very dark place, you can see a faint, milky band of light stretching across the sky from near north to south. This is our home galaxy, the Milky Way, called Ākāśha Gangā in India. A galaxy is a huge group of millions to billions of stars. Our Sun is just one of the stars of the Milky Way.

The Universe. Beyond the Milky Way there are countless other galaxies. All of them together make up the Universe.

Is there life elsewhere? We do not know yet. Scientists are searching, mostly on exoplanets, planets that go around other stars in our galaxy. No evidence of life has been found so far, but the search goes on. Perhaps you will help answer this question one day.