Similarly, if you go half the initial distance, the Sun's gravity becomes four times stronger. As a result, Mercury has the greatest gravitational pull to the Sun. (This is also why it circles quicker; imagine spinning a rock on a thread with a short string: rapid spin=more force!)
The reason Earth gets a head start is because its mass is so much greater than those of the other planets. If Mars or Venus had Earth's mass, then they would experience an equal amount of force from the Sun's gravity, and there would be no reason for them to move in elliptical orbits.
Earth's mass is about 5% that of the Sun, so its gravity pulls it away from the Sun at about 5% of the speed of light. This isn't very fast, but it's enough to keep us living on its surface!
The other planets don't get this lucky break and so their respective moons are pulled towards the planet at significant speeds. Enceladus' moon Dione travels around it at more than 20 km/s, while Saturn's Tethys moves at only about 10 km/s.
These speeds were calculated using math called "angular momentum", which is just another way of saying "rotation".
Mercury, as you might expect given its speed, is the planet nearest to the sun and hence the most influenced by its gravity as it zooms around and about. At an average distance of 40 million km (25 million miles), even Venus, which is closer than Mercury, cannot outrun the speed of light when it comes time for it to leave Earth and head towards the sun.
However, because it spins on its axis so quickly, one day on Mercury is equivalent to nearly two days on Earth. This means that although it isn't visited by meteorites or satellites, some things get done on Mercury that wouldn't be possible otherwise. For example, since it's impossible to land a spacecraft on the planet, scientists have used images from probes sent to it by orbiting telescopes.
Landing on Mars would be difficult because it takes more than three months for messages to reach it at its current distance of 140 million km (87 million miles). But if it were any closer, it would be within the orbit of Mercury.
Venus is often described as the Earth's twin, but this analogy breaks down when you consider how different they are. Venus is almost always over 100 degrees Celsius (212 degrees Fahrenheit) and almost always cloud covered. It has no surface water, nor has there ever been evidence of any.
Mercury must have a higher orbital velocity than Earth since it is nearest to the Sun. This is why Mercury is the planet with the greatest speed in relation to our sun. This is also why, when a planet moves away from the sun, its orbital velocity decreases (as long as the orbit is near circular, if not, circular).
Earth's orbital velocity is about 30 km/s, while Mercury's is 55 km/s. This means that it takes Mercury only 987 km/s to orbit the Sun instead of 365 days 6 hours 45 minutes like Earth's 780 km/s.
This high orbital velocity is due to Earth's proximity to the Sun. Our planet is always radiated by the heat of the Sun so it cannot freeze over and we would be destroyed by this heat. But because of its distance from the Sun, Earth's average surface temperature is 4.18 °C. Mercury, which is closer to the Sun, has an average surface temperature of 48.9 °C!
Of all the planets, Mercury is the most important for humans because we live on Earth and we experience everything from human life because all of these events occur on the inside of Mercury's orbit. Before astronomers discovered other planets, people knew about Mercury because it was the closest planet to the Sun, and thus it could be seen easily with the naked eye.
In addition to being close to the Sun, Mercury has many features that make it interesting to study.
Which of the following best depicts why gravity on Earth has a stronger pull on you than the sun? Although Earth has a lower mass than the Sun, it is far closer to you, allowing for a stronger pull.
Gravity on Earth gets a strong reaction from matter because all matter emits some type of radiation that tells physics where it is in relation to other matter. This means that if there were no radiation coming out of a person's body, we would know they weren't touching anything else with their mass, which wouldn't be good since they'd be floating around in space without any way to stop themselves.
The reason Earth's gravity has a stronger attraction on you than the sun's is because the sun is so massive that even though it is very far away from you, its gravity is still strong enough to keep most celestial bodies near it. But on Earth, due to our proximity, even though it is a much smaller planet, its influence reaches all the way down to your location.
This is why astronauts experience weightlessness during orbitals around Earth-they are outside of the influence of Earth's gravitational field. And since they are not close to any other large object with mass, they can float around in space with nothing but a few microns of nylon to keep them afloat.
Mercury is the planet closest to the Sun and the most difficult to view since it is usually in the sky relatively close to the Sun. It seems to swing back and forth around the Sun due to its orbital motion, reaching a maximum angular distance of around 28 degrees.
The best time to observe Mercury is when it is high in the sky and near the horizon. It must be far from the Sun because otherwise it would be too bright to look at. The best place to observe from is probably away from city lights so that you can see all the way out to the horizon.
You can use your telescope to see Mercury even though it is so close to the Sun. Just point it in the direction that will bring Mercury into view and watch it move across the face of the Sun. You should see four distinct dark spots on its surface where it has turned toward the Earth. These are called faculae and they make up more than half of Mercury's surface area. The other half is deep space, so very little of it is illuminated by the Sun at any one time.
Faculae appear as small white or yellow patches because they reflect sunlight like a mirror does but with the difference that the mirror only reflects light waves that have a horizontal component. So only part of the radiation from the Sun reaches the surface; the rest passes over the edge and into space.