When seen via a telescope, both planets exhibit phases. The planet is in its full phase and has a relatively tiny angular diameter when it is in superior conjunction. As Mercury or Venus orbits the sun, the phase shifts from half phase to crescent as it approaches inferior conjunction. At this point, the planet is fully illuminated from one side only as it travels across the face of the sun.
This is because both planets are tidally locked to each other. That is, they rotate around their axes at exactly the same rate, so that the same part of each rotates toward and away from the sun during an orbit. Because the planet Earth also follows this pattern, people on both sides of the planet experience night during certain periods of planetary rotation. It is only when the planet reaches either superior or inferior conjunction that another portion of itself can be exposed to view.
In addition to these two factors influencing how much of the planet is visible at any given time, the angle at which it is seen from Earth also plays a role. If Earth was directly facing away from the sun, none of the planet would be visible. But because we are looking towards the sun, some parts are always illuminated.
Planets take a few days to complete one rotation, but because of their proximity to the sun, they speed through their orbits quite quickly - sometimes more than 100 times faster than Earth!
When compared to the other planets, Venus and Mercury have the most visible phases. This is due to their respective orbits around the Earth. Mercury and Venus are both inferior planets, which means they orbit closer to our parent star than Earth does. As a result, they experience more heating from sunlight than do the other planets.
Venus and Mercury are also terrestrial planets, which means that they resemble Earth in many ways. Both planets have days and nights, clouds, gravity, and even surface features such as oceans, continents, and volcanoes. The only thing missing from their list of qualities is a moon. They're too small to have any impact on each other's surfaces, but this doesn't stop them from being interesting from a scientific perspective!
The reason for Venus and Mercury's appearance to be so different from one another is because of their distances from the Earth. From our point of view on Earth, the closest planet to us is Venus; however, from the vantage point of Venus itself, it appears far away because it is so small when compared to the distance between itself and the next-closest planet, Mercury. Same with Mercury, which appears very large because it is so close to us.
This same effect has been used by astronomers to discover new worlds beyond our solar system.
An inferior planet located on the same side of the sun as Earth appears dark and difficult to see. On the other side of the sun, we see the planet virtually fully illuminated, with crescents of varying sizes in between. Snapshot 1: Both Mercury and Venus are in their crescent phases. The only problem is, they face opposite directions from our viewpoint on Earth. From where we stand, Venus is obscured by clouds while Mercury is always dark because it is too far away from the sun for its atmosphere to retain any heat.
Venus and Mercury have been in this phase for about three weeks each. As you can see, only the half of Venus that is facing Earth is visible. The rest is hidden from view by the sunbeam filtering through our atmosphere. From Venus, however, all of Earth is exposed.
Earth also goes through this phase every night, but since it happens when the moon is out, it's rarely seen from Earth. However, if you were standing on Mars, you would see the entire disk of Earth going through its monthly cycle of light and darkness.
Mars has two moons: Phobos and Deimos. They are both small (about 50 miles or 80 kilometers across) and irregularly shaped, probably formed by collisions with smaller bodies. Phobos orbits around Mars every 687.4 hours, whereas Deimos takes 998.5 hours to complete one orbit.
Which Planets Have Phases That Are Similar to Lunar Phases?
They occasionally appear to cross the solar disk, which is known as a "transit of the planet." These planets display the whole spectrum of crescent and gibbous phases at intermediate times in their orbits. Superior planets, which revolve outside the Earth's orbit, do not display the whole range of phases, appearing almost invariably as gibbous or full. Inferior planets, which lie inside the Earth's orbit, exhibit all phases of the moon but tend to be more visible when near the point of maximum distance from the Earth.
Planets don't always appear in the same part of the sky to everyone - some people may see them east of south, while others see them west of north. But they always appear somewhere within an arc just above the horizon on either side of the zenith. So if you look out for them, you should be able to spot them anywhere within these arcs. Of course, where there are many stars there will also be many shadows, so keep that in mind when trying to find a planet against the night sky!
In addition to these observable effects, each planet also causes a unique pattern of darkening on the sun during a transit. The darkness varies depending on the size of the planet and its proximity to the sun. Smaller planets like Mercury and Venus completely cover the face of the sun during their transits. Jupiter, on the other hand, only dims it slightly, while Saturn and Neptune are too far away from the sun to affect it at all.