As a result, when Mercury is observed from Earth on one side of the Sun, the same side of Mercury is facing the Sun, and when it is seen from Earth on the other side of the Sun, the same side is facing the Sun. This phenomenon is called "retrograde."
Mercury goes through its orbit around the Sun in 88 days; during that time, it moves almost exactly half way around the sky from east to west. Because of this motion across the sky, different parts of Mercury are visible from Earth at different times. In fact, if you were to watch Mercury every day for an entire year, it would go through all four of these phases.
When Mercury is first seen after sunset, it is rising in the east and setting in the west. Then it reaches its highest point in the sky, where it remains for about two weeks. Finally, it begins to set as it enters the western part of your night sky. All together, these four phases cover nearly all of Mercury's orbit around the Sun.
The reason why we only see one half of Mercury each time it passes behind the Sun is because the other half is still too far away from us. Even though Mercury is closer to the Earth than any other planet, it still takes 8 minutes 40 seconds for messages to travel between them.
From Earth, Pierre Gassendi uses a telescope to see Mercury as it passes the face of the Sun. 1965: Using radar, astronomers discover that Mercury spins three times per two orbits, contrary to millennia of wrongly assuming that the same side of the planet always faces the Sun.
Mercury was discovered in April 0-566 AD by the Roman astronomer Marcus Aurelius Antoninus. The reason for its discovery is because it wasn't there the previous day.
It took another 200 years before this news was published in Europe. By this time, several other people had seen Mercury crossing the Sun but only Gassendi's notes are still available today. They are contained in a book called "Astronomical Observations Made in France", written by Guillaume Le Gentil de la Lande (c. 1594 - 1669). La Lande was the director of the Paris Observatory from 1646 to 1669. He made many significant observations during his career which has led him to be regarded as the father of French astronomy.
Later he went on to say that he had also seen a second object follow a similar path a few minutes later.
Mercury, like Venus, circles the Sun as an inferior planet inside Earth's orbit, and its apparent distance from the Sun as seen from Earth never exceeds 28 degrees. Because the planet is so close to the Sun, it can only be seen at the western horizon after sunset or the eastern horizon before sunrise, generally in twilight. When this occurs, observers on Earth see a crescent Moon when viewed from locations where it is still night.
The most common explanation for this phenomenon is that like Earth, Mercury has an iron core, which causes it to possess a magnetic field. This magnetic field causes particles in Earth's atmosphere, such as electrons, protons, and ions, to move along magnetic lines of force in a process called "magnetism." The Sun's own magnetic field causes these particles to rotate around it, just like the planets do. As these particles cross the face of the Moon they collide with it, leaving craters. Because Mercury has a magnetic field, these particles on Mercury follow magnetic lines of force across the surface, creating a pattern of high and low regions called "latitudes."
The impact theory explains why there are few large mountains on Mercury. It also explains why there are no oceans: The heat from the Sun would evaporate any water that may have existed under Mercury's surface. The collision theory explains why there are no signs of any past asteroid impacts.
Closed three years ago. The Sun, according to the Globe Earth Theory, is at the center of the solar system, and the Earth is the third planet from the Sun, leaving Mercury and Venus as the only planets between Earth and the Sun. If this is correct, Mercury and Venus should be seen exclusively from the illuminated side of the planet, as they are constantly between the Sun and the Earth. However, both these planets can often be seen in the evening sky after sunset, especially when viewed from a low-latitude region where they are closer to the horizon.
Open since 1846. The Earth's atmosphere allows certain distant objects to be visible to the human eye. Astronomers call these objects "astronomical bodies." The word comes from a Latin phrase meaning "the stars." Our galaxy has hundreds of billions of stars, but only eight planets including Earth. So almost all of them are lost in darkness, unless they cross our path or the path of one of their moons. The exceptions are those objects that reflect light from the sun back toward earth: Mars, Jupiter, Saturn, Uranus, and Neptune. They are called "solar systems" because they contain star(s) similar to our own.
Our galaxy has 100 billion galaxies, each with hundreds of billions of stars. So there are definitely some other astronomical bodies out there that cannot be seen with the naked eye. But since they are so far away, we need telescopes to see them.