Planet Earth is the most beautiful image a person could look at, it inspires awe and wonder. For much of human evolution almost all knowledge has been restricted to everything about the Earth and everything else out there was quite frankly conjecture and models were based at first on myths until telescopes came along and then theorise on the precise measurements of discovered planets in the solar system.
The universe it is an unimaginably big thing to comprehend, perhaps the biggest concept humans will ever have to study. Thankfully on this page we will confine our wonderment to one planet, which we call home. There is only one habitable planet we know of that has the abundance of life and as such we’ve come to regard it as a near miracle. We have not found anything like it in the observable universe.
“Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives.
“Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.“
— Carl Sagan
The photograph above is very famous in astronomy, known as pale blue dot. It was taken by NASA’s Voyager 1 spacecraft in 1990 and that barely visible dot in the middle to the right is Earth captured from Voyager’s position approximately 32° above the ecliptic. Very surprisingly comments on websites have slated this image as insignificant, out of focus and pointless but I hope you will agree that this single dot, which comprises just 0.12 of a pixel out of the 640,000 in the image, is an amazing testament to human exploration.
To get the image took plenty of red tape breaking at a time NASA was involved in other high priority space missions like Magellan and Galileo, especially as the intended photo had no scientific value considering the poor quality that would result as a consequence of distance, the capability of the onboard cameras and the affect from Rayleigh scattering.
The optical phenomenon of Rayleigh scattering is a scattering of electromagnetic radiation that applies to particles that are smaller than the wavelength of light. Light acts on these particles causing them to vibrate at the same frequency and we see scattered light. It is the main effect that shows the Earth in a blue colour from space.
Rayleigh scattering in the Earth’s atmosphere is the cause of the Earth’s blue hue where short wavelength visible light, like blue, is scattered over long wavelength visible light, like red. If it were the other way around the Earth would appear red seen from space. The large sea mass contributes to Earth’s blueness to a lesser degree than Rayleigh scattering but light reflected from clouds combines with the scattered blue light to make the tone value paler. In fact the wide range from untraviolet to infrared is not observed anywhere else in the galaxy which is partly down to life on Earth where photosynthetic life forms release oxygen and make the atmosphere transparent so that substantial Rayleigh scattering occurs.
The banding seen on the image is due to sunlight reflecting off the camera and its sunshade. Even though the camera is primitive by today’s standards, laboratory analysis has revealed that the moon is also present, so faint it can’t be seen with the eye. Even more astonishing to think the image was taken four billion miles from Earth, that’s 40472229 astronomical units.
Voyager 1 was only scheduled to operate for around four or five years and when it completed its mission, it continued to head out of the solar system. At least back then the edge of the solar system was thought to be not much further than the last planet Pluto. We have since discovered that this assumed distance is in fact just 10% of the sun’s gravitational influence.
But in any case, in the day, permission was finally approved to have Voyager 1 take the photo of Earth, at the request of astronomer Carl Sagan, after its mission was done. One of its two cameras was turned back to Earth having first calculated the length of exposure and moment at which best to capture the shot so as to protect the vessel’s fragile imaging circuitry. It was taken at 04.48 GMT on 14 February 1990. But it would be several weeks before the data could be scheduled to return to Earth. It was Carl Sagan that coined the phrase ‘Pale Blue Dot’, in his book of the same name in 1994.
In 2020 NASA released a remastered photo. It was not significantly altered in respect for the original image, however the two images do look very different.
Earth from Galileo
The Earth and Moon captured by NASA’s Galileo spacecraft after its second encounter with Earth on its way to Jupiter. Galileo looked back to capture this image on 16 December 1992, at a distance of just under four million miles. It reached the Jovian system in 1995-97.
ESA – Earth from Moon
This is the classic “Earthrise” photograph, captured by astronaut William Anders during the Apollo 8 mission – the first human mission to the Moon. This image was taken on 24 December 1968.