Which celestial object used to be known as both the morning star and the evening star?
And the answer: Venus.
As the second planet from the sun, Venus passes closer to Earth than any other planet. From Earth, it's one of the brightest objects in the sky. In the 5th century BCE, the Greek philosopher and mathematician Pythagoras was the first to realize that this bright object in the morning and the evening was the same object.
Unlike most other planets in our solar system, Venus orbits on a very similar path to Earth. No planet approaches closer to our planet than Venus, and because of its proximity to the Sun, it is often seen in the same direction during the hours of sunrise or sunset. For this reason, the Ancient Greeks and Egyptians perceived Venus to be two separate celestial objects (and thanks to Venus' luminosity, they thought it was a star – hence the name).
Venus was one of the five planets known in ancient times, alongside Mercury, Mars, Jupiter and Saturn. In fact, the name "Venus" comes from the Roman goddess of love and beauty (perhaps more familiarly known to us as Aphrodite in Greek mythology).
Despite earning the common perception as "Earth's twin," the two could not be more different. Yes, they are of a remarkably similar size – all the way down to nearly equal planet density – but Venus is an incredibly hot planet incapable of hosting life. At a surface temperature of 900 °F (482 °C), the pressure of its carbon dioxide atmosphere is 95 times that of Earth’s atmosphere, and its clouds are made of sulfuric acid. Recent studies interrogate just how it could be possible for two neighboring, similarly sized planets to be so intensely different.
Learn more about Venus here.