The first 5 planets of our sensational solar system are very hard to date, being visible to the naked eye meant they were all identified long ago. Of course it does depend on their distance and whether you count actual up-close sightings, but the discovery date is unknown, what we do know though is their initial recorded date. The planets have changed over time and each one is like an elder of the sky, watching over every star, organism and speck of dust in the whole solar system. Here we’re going to go through the date of each planet’s discovery, whether written or actual, and some fun facts about each wonder.


As I said earlier, due to its positioning and visibility to the naked eye, Mercury doesn’t have a definitive discovery date. The Mul.Apin tablets hold the earliest known recorded observations of the planet and were likely made by an Assyrian astronomer. The tablets themselves are thought to date back to 1000BCE or earlier, they contain the largest surviving record of Babylonian stars and constellations. The cuneiform name (cuneiform being a type of writing used in Mesopotamia, Persia, and Ugarit) used on these tablets for Mercury is Udu.Idim.Gu4.Ud or “the jumping planet”. Later on, the Romans called the planet Mercury after their messenger god as it moves across the sky faster than any other planet.

Mercury’s temperatures reach a rather sweltering 840°F (450°C). It’s 18 times smaller than Earth with a diameter of 3,031 miles (4,878 km) and goes around the Sun every 88 days, one day on Mercury lasts just under 59 Earth days. Mariner 10 was the first to visit Mercury flying by in 1974 and 1975, seeing less than half of the planet. After that no more was seen for over 30 years. Then in 2008 NASA’s MESSENGER spacecraft flew by and in 2011 it began orbiting the planet sending photos back to Earth.


The first written observation of Venus is a record spanning 21 years of its appearance in the sky, written on a Babylonian text from 1600 BC, it’s also one of the oldest surviving astronomical documents. Galileo Galilei in 1610, the first person to observe Venus through a telescope, noticed the planet goes through Moon-like phases, this supported the Copernican view that the planets orbit the Sun, and not the Earth as earlier thought.

The surface of Venus reaches 900°F (480°C), Venus has no known moons and is 20% smaller than the Earth. One year on the planet equates to 225 Earth days. The symbol of Venus is also that of the female, the ♀, alongside this, all but 3 of Venus’ surface features are named after famous women from around the world, the other 3 are Alpha Regio, Beta Regio, and Maxwell Montes.


Of course Earth wasn’t discovered, apart from each individual’s discovery upon entry into this green and blue spinning glory. The first Photo of Earth from space was captured by a V-2 test rocket on the 24th October 1946. The spherical nature of Earth was previously the cause of many heated debates, the idea was mentioned in Greek philosophy in the 6th century BC but until the 3rd century BC it was nothing more than speculation. Hellenistic astronomy backed up the concept and up until the Middle Ages the idea wriggled itself into people’s beliefs. Ferdinand Magellan and Juan Sebastián Elcano’s round the world voyage between 1519 and 1522 was conclusive proof of the Earth’s spherical nature.

As we all probably know the Earth has 4 layers; the crust, mantle, outer core and inner core, and is 70% water. However, more interestingly, if Earth was the size of a nickel, the sun would be a front door.


The first recorded observations of Mars were from ancient Egyptian astronomers in the 2nd millennium BC. In around 300 BC, Aristotle weighed in, noting that Mars is actually further away than the moon after noticing the moon passed in front of mars, proving of course it must be closer.  Galileo once again possibly had the first telescopic view of Mars between 1608 and 1610, but Francisco Fontana made the first actual records in 1636. His sketches showed more the poor quality of telescopes of the day rather than any real depiction of Mars. Christian Huygens probably made the first informative Mars sketch in 1659.

In 1965 22 photos of Mars, the first close-up photos of any other planet outside Earth, were sent back by NASA’s Mariner 4. Mars has a surface temperature averaging -81°F (-63°C) and is around 50% the size of Earth and has 38% of the Earth’s gravity.

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