Why can we see the sky in the course of the day, however the galaxy at night time? — Gary, age 9, Auckland
Hello Gary! Thanks for this nice query.
To place it merely, the explanation the sky appears to be like completely different to us between daytime and nighttime is usually due to our ambiance.
The ambiance surrounds Earth, and extends from the bottom to outer house. It’s made of various gases together with oxygen (which retains us alive as we breathe it in), carbon dioxide (which we breathe out), methane (which can also be in our farts), nitrogen and argon.
All these gases, in addition to all solids and liquids, are fabricated from molecules. Molecules are collections of atoms that are a lot too small for us to see, however are the fundamental constructing blocks of every part that exists. Importantly, completely different molecules have completely different combos of atoms and are available in several sizes.
A molecule’s dimension performs a job in how the molecule interacts with gentle. Gentle from the Solar isn’t one color — it’s made up of all the colors of the rainbow (which is why we see a rainbow when gentle behaves in a sure approach).
A few of the molecules in Earth’s ambiance are simply the appropriate dimension that the blue a part of the sunshine from the Solar bounces off them, scattering in several instructions.
So once we look in the direction of the Solar in the course of the day (remembering that you need to by no means look immediately on the Solar), we see rays of sunshine which have come from the Solar straight all the way down to us.
However once we look away from the Solar we see the blue gentle rays scattering from the a part of the ambiance we’re . That’s why the sky is so brilliant — and blue — in the course of the day.
The galaxy at night time
At night time we see stars in a darkish sky, and these stars make up our galaxy, the Milky Approach. The Milky Approach is fabricated from an enormous variety of stars, together with the Solar.
In contrast with different stars, the Solar truly isn’t that brilliant, but it surely appears to be like very brilliant to us as a result of it’s a lot nearer to Earth than different stars are.
At night time, when your facet of Earth is dealing with away from the Solar, the one gentle that reaches you is from different stars. This starlight additionally scatters off molecules within the ambiance, however as there’s much less of it, not a lot scattering goes on.
Because of this, at night time once we’re dealing with away from the Solar, we don’t see the identical factor as when we face the Solar in the course of the day. As a substitute, we will look via the ambiance and past on the massive, darkish expanse of house round us and the numerous, many faraway stars in our galaxy.
Curious Youngsters: how does the Solar make such fairly colors at sunsets and sunrises?
The sky on different planets
Different planets in our Photo voltaic System have completely different molecules of their ambiance in contrast with Earth, which suggests their skies look completely different in the course of the day and night time.
The ambiance on Venus, for instance, is so thick you’ll by no means see the Solar — not even in the course of the day while you had been dealing with in the direction of it. The celebs aren’t seen at night time, both.
Astronauts who’re above our ambiance, resembling on the Worldwide Area Station or on the Moon, don’t see the intense blue sky we see on Earth. As a substitute, they see the Solar as a big close by star in opposition to a black sky.
They usually can see the galaxy on a regular basis.