Facts for kids: What is a light-year?

What is a light-year?

There’s a good reason we say the Andromeda Galaxy is 2.5 million light years away

A light-year is a measurement of distance, often used in astronomy to express how far light travels across space in a standard Earth year of 365.25 days. Because light travels so fast – 300,000 kilometres (186,000 miles) per second – numbers get too messy when dealing with galaxies and stars beyond the solar system.

One light-year is equal to 9,500,000,000,000 kilometres (about 6 trillion miles). Or, to be more precise, one light-year is 9.460730472580800 trillion kilometres (5.878625 trillion miles).

When considering Proxima Centauri, the nearest star, it’s a lot easier to express the distance as 4.243 light years as opposed to 40,018,889,899,016.78 kilometers. Would you rather say the Andromeda Galaxy is 2.5 million light years away or 23,651,826,181,452,000,000 kilometres away?

Short answer: A light-year is a measurement of the distance light travels through space in a year. It is often used to express distances beyond the solar system.

Facts for kids: What causes thunder and lightning?

Thunder and lightning

Lightning hitting a tree.

Thunder and lightning is a fascinating phenomenon fuelled by the interaction of subatomic particles. But let’s keep it simple. Thunderstorms begin their lives in cumulonimbus clouds. These clouds are formed from water vapour carried on rising air currents, often reaching an altitude of about six kilometres (20,000 ft). At this height the air is colder than it is at sea level. Drops of water, hailstones and specks of ice form and collide with each other. This is where the magic begins.

An electrical charge is created within the cloud. The top of the cloud takes on a positive charge as protons rise, while electrons descend to the bottom of the cloud. The negative electrical charge at the bottom of a cloud creates a positive charge in the ground underneath it. As the negative charge gets stronger, the air cannot stop it from jumping from the cloud towards the positive charge on the ground.

The result is a giant spark of electricity, also known as lightning.

Thunder is the product of lightning. The temperature of a lightning bolt reaches up to 30,000 C (54,000 F), or about six times hotter than the surface of the sun. The bolt heats the air around it causing it to expand and then contract. The result is a shockwave that you hear as thunder.

Short answer: Lightning is a flash of light created by electric discharge. Thunder is a soundwave created by the rapid expanison and contraction of air.

Facts for kids: What is the oldest known rock?

The oldest known rock

The oldest known rock was formed during the Hadeon.

The oldest known rock on Earth is located in Western Australia. Scientists recently discovered an ancient crystal called a zircon and dated it to around 4.374 billion years old. Because erosion carves the land, continents drift, volcanoes erupt and seas rise and fall, finding any evidence of early Earth is quite difficult.

The crystal hails from a time known as the Hadean Eon. The Earth was highly volcanic, had a partially molten surface and was frequently struck by meteors, asteroids and comets. The planet was lifeless and extremely violent.

The zircon itself isn’t much to look at. It’s about the size of a house dust mite and located inside an existing rock. It’s not exactly the kind of thing you’d find on a casual walk in the bush.

Short answer: The oldest known rock comes from Australia and is nearly 4.4 billion years old. In comparison the Earth is around 4.54 billion years old, and the moon nearly 4.5 billion years old.