Ground-based Telescopes
Most telescopes used by astronomers are on Earth. We call these ground-based telescopes.
Building a telescope on Earth is much easier and cheaper than in space. It is also much easier to fix if things go wrong.
![A mountain top with a number of white domes that house telescopes. There are also a few small buildings. In the background, the sky is light blue towards the top, and there are some white clouds lower down, especially below the mountaintop.](/sites/default/files/styles/things_to_do_photovidppt/public/1024px-La_Palma_-_Garaf%C3%ADa_-_Roque_de_los_Muchachos_Observatory_-_ATC_-_WHT_%2B_LT_%2B_DOT_%2B_MT_%2B_SST_%2B_INT_%2B_JKT_%28Mirador_de_Roque_de_los_Muchachos1%29_01_ies.jpg?itok=zQa2-dKh)
Brightness
Light is the main signal we receive from space. Almost everything we know about space has been found by measuring light.
One of the main ways to measure light is to look at the brightness. Measuring the brightness of something in space isn't as easy as you'd think though.
![A dense star cluster filled with thousands of bright stars, some shining with a blueish hue, while others emit a warmer orange glow, set against the blackness of space.](/sites/default/files/styles/things_to_do_photovidppt/public/cluster.jpg?itok=b9J6CwUW)
Electromagnetic Spectrum
This light that we see is just one small chunk of light which is made by stars. We often call the small range of wavelengths our eyes can detect, optical, or visible light.
![A black arrow cuts diagonally downwards across the image towards the lower left, acting as an axis for a blue vertical wave labelled "electric field" and a red horizontal wave labelled "magnetic field". These waves are at right angle to each other. An arrow with heads at either end spans the length of the top of a vertical wave to the top of the next one.](/sites/default/files/styles/things_to_do_photovidppt/public/Electromagnetic_wave_EN.svg_.png?itok=2HqszsoG)
Wave-Particle Duality
Light is complicated as it can act as both a wave and a particle. Choosing how we want to talk about light - wave or particle - is only a matter of convenience.
![An image of the gamma symbol](/sites/default/files/styles/things_to_do_photovidppt/public/Gamma_animiert_2018.gif?itok=eLwBeJyy)
Spectra
Even though light often looks white, it is made up of lots of different colours all added together.
![White light from the bottom left of the image travels upwards towards a glass prism in the centre. Some light is reflected, continuing to travel towards the centre top of the image. Most of the light leaves the prism and travels towards the bottom right of the image, now split into the different colours of the rainbow.](/sites/default/files/styles/things_to_do_photovidppt/public/Dispersive_Prism_Illustration.jpg?itok=colKIush)
Light
When we talk about light we usually mean the light we can see with our eyes.
This is also called visible or optical light.
Light is one way energy can be transferred - moved from one place to another.
![A bright sun at the top of image, slightly off-centre, shines down over tall, thin trees with green foliage. The sky is bright blue with some white clouds visible.](/sites/default/files/styles/medium_500px_wide/public/800px-A_few_trees_and_the_sun_%286009964513%29.jpg?itok=dDHwExsZ)
Magnetism
Since ancient times, people have known about some rocks that could attract iron. When these rocks were rubbed on other types of metal, they had the power to make them attractive too. These items could also push each other away, repelling each other.
![A dark grey and brow rock with a textured surface. On this, three metal nails are sticking to it.](/sites/default/files/styles/things_to_do_photovidppt/public/52558585533_58183b2c0e_b.jpg?itok=GS0nbKb7)
Climate Change
The Earth's climate is always changing. It shifts through a regular cycle of temperatures. These cycles last around 100,000 years. They move the Earth through warm periods, then cold ones. This is known as climate oscillation.
![An infographic illustrating changes in average temperature, displayed in Fahrenheit. The graphic uses color gradients and numerical values to represent temperature shifts over time, indicating warming trends](/sites/default/files/styles/things_to_do_photovidppt/public/Change_in_Average_Temperature_With_Fahrenheit.svg_.png?itok=8r62lLq3)
Light and Telescopes
One of the first people to study light was Ibn al-Haytham (known as Alhazen). He was a Muslim, living in Egypt in the early 11th Century. Alhazen is first person we know of who used the scientific method. He wanted to know why our eyes could see things.
![A simple experiment using paper and a light source to show light travels in straight lines..](/sites/default/files/styles/things_to_do_photovidppt/public/learn/Light%20Lines.png?itok=_j4Hy7u3)