Then it continues its regular westward migration, and as Mercury gets closer to the Sun, the size of the solar disk shrinks until sunset, 88 Earth days after dawn. What a wonderful day! Then, for the next 88 Earth days, it is night for you. The planet's orbit is almost entirely within the Sun's gravitational pull, so one side is always dark.
Mercury has only two seasons: summer and winter. Ice covers most of the planet for most of the year, but small areas near the equator thaw out when the sun warms them from below. The remaining ice protects these regions from the heat and pressure of sunlight that would otherwise destroy the surface.
During summer, the sun is directly over Mercury's center of mass, so all points on the planet receive an equal amount of light. But because the planet lacks any significant atmosphere or magnetic field, most of this energy is reflected back into space. Only a small fraction reaches the surface.
The average temperature of Mercury is -150 degrees Fahrenheit. All the water on the planet may be frozen, but there is still enough energy circulating through the planet that life could exist in the form of volcanoes and aquifers.
The average distance between planets in the Solar System is about 12 million miles (19 million kilometers). On Mercury this comes to 63 miles (101 km), which is very close.
The Earth revolves on its axis as it circles the Sun, and one full revolution takes around 24 hours. At any one moment, the sun illuminates one half of the Earth, causing it to experience daylight, while the other half experiences nightfall. As the Earth rotates, each location will go from day to night and back again. During a single day, the same region may experience several transitions from dark to light and back again.
In addition to daily changes in illumination level, certain locations on Earth have seasons that result in permanent changes in climate. The two main factors responsible for these seasons are solar radiation received by Earth's surface and distance from the Sun. Solar radiation increases with altitude, so higher regions receive less heat from the Sun than lower regions. Regions far away from the Sun experience cold winters and hot summers; these are called "cold" and "hot" climates, respectively. Less distant parts of this orbit experience both extremes, but more often than not, locations that are close to the Sun are also warm temperatures all year round.
Half of the world experiences daylight and nighttime. This is because over half of the Earth is covered in water, and most of the water is ocean. The ocean covers 3/4 of the Earth's surface, and so almost every point on the ocean has sunlight at some time during the year. Only a small part of the ocean is close enough to land that there's no sunlight during the whole year - the polar regions.
Mercury completes five cycles around the sun in 440 days. It takes 87.5 years for mercury to make one full rotation around the sun.
The average distance of Mercury from the Sun is 59 million km (37 million miles). At this distance, sunlight strikes the planet at a rate of 500 watts per square meter (5,778 watts per square foot). This is more than enough energy to evaporate water and cause other changes on Mercury's surface.
However, because Mercury has no atmosphere or magnetic field, most of this radiation is directly absorbed by the surface instead of being reflected away like it is on Earth. As a result, much of the power is lost as heat, which accounts for why Mercury appears so cold at both poles. In addition, because there is no air to carry away heat, much of the surface temperature can reach hundreds of degrees Fahrenheit (100 degrees Celsius).
This may seem like a bad thing, but it provides evidence that the planet was once warmer inside its own mantle. Currently, the hottest place on Mercury is the polar cap, which can reach 900 degrees Fahrenheit (500 degrees Celsius).