Solar sketching



This detailed solar observation, comparable to modern photographs, was drawn by the astronomer Samuel Pierpoint Langley in 1873. It shows two sunspots – darker, cooler regions of the solar surface – surrounded by cells of hot gas the size of Earth's continents, rising up from the sun's interior.



Sunspots are caused by intense magnetic activity and usually herald an increase in solar flares and coronal mass ejections. These can disturb Earths magnetosphere, interfering with radio communications, disrupting navigation and boosting auroral activity. In 1989 a solar storm caused a major electrical power blackout in Quebec, Canada, affecting millions.



Nowadays we can observe the sun in incredible detail using, for example, a NASA probe known as the Solar Dynamics Observatory (SDO). This should help us understand the causes of solar variability and its impacts on Earth.



(Image: Royal Observatory, Greenwich)

Sizing the solar system



Early astronomers recognised that transits of Venus – short periods when our neighbouring planet passes directly between Earth and the sun – could give us a way to measure the size of the solar system.



However, transits of Venus are incredibly rare: although they occur in pairs eight years part, each pair is separated by more than a century. They were observed during the 18th century, including on Captain Cook's voyage to Tahiti, but the quality of data wasn't good enough to do the mathematics.



By the time the next pair of transits came round in the 19th century, astronomers had a new weapon in their arsenal: photography. Capturing images like this one, taken under the clear skies of Luxor in Egypt, helped astronomers calculate how far away Earth is from the sun.



(Image: Royal Observatory, Greenwich) Advertisement

Dark side of the moon



The moon is a familiar sight – at least, the face permanently turned towards Earth is. In 1959, the Soviet Luna-3 probe gave us a new perspective when it sent back this grainy picture of the lunar far side. It looks different from the near side as the crust here is harder and cooler.



Subsequent missions have discovered an abundance of water located at the moon's poles.



(Image: National Space Science Data Center (NSSDC) Photo Gallery)

Cold weather



Uranus, the seventh planet from the sun, was discovered in 1781 by astronomer William Herschel. Its axis of rotation is unusually close to the plane of its orbit, leading to unusual weather systems as it spends long periods with one hemisphere facing the sun and the other plunged into darkness.



The planet has the coldest atmosphere in the solar system, at around -200 ºC. It has only been visited once by a spacecraft, when Voyager 2 swung past in 1986, but remote study has shown us a lot. This infrared image from 1998 was taken by the Hubble space telescope, which can detect the faint thermal glow from the planet and its four major rings, as well as ten of its attendant moons. It shows the temperature variations on Uranus, with bright spots marking the site of huge storm systems.



(Image: NASA/JPL/STScI)

Water on Mars



With spacecraft orbiting it and two rovers currently active on the surface, Mars is one of the most studied bodies in the solar system. Researchers want to pore over its diverse landscapes for evidence that the Red Planet may once have enjoyed conditions suitable for life



Here, NASA's HiRISE camera on board the Mars Reconnaissance Orbiter looks down on complex geological formations where erosion has revealed intricate layers of sediment. These may give clues to a watery period in the distant Martian past.



(Image: Malin Space Science Systems/MGS/JPL/NASA)

Maximum resolution



This picture shows the Orion nebula, the nearest star-forming region to Earth, in unprecedented detail. It is a mosaic of 520 individual Hubble images, taken in five colours, supplemented with images from the European Southern Observatory's 2.2-metre telescope at La Silla in Chile. More than 3000 stars are visible at various stages of birth and development. Containing a billion pixels at full resolution, NASA's image shows how far astronomical imaging has come in 130 years.



(Image: NASA/ESA/M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)