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The second planet from the Sun is Venus. After the Moon, Venus is the most brilliant natural object in the nighttime sky. It is the closest planet to Earth, and it is also the most similar to Earth in size, mass, volume, and density. These similarities suggest that the two planets may have had similar histories. Scientists are thus intrigued by the question of why Venus and Earth are now so different.

in timekeeping

3,600 seconds, now defined in terms of radiation emitted from atoms of the element cesium under specified conditions. The hour was formerly defined as the 24th part of a mean solar day—i.e., of the average period of rotation of the Earth relative to the Sun. The hour of sidereal time, 1/24 of the Earth’s rotation period relative to the stars, was about 10 seconds shorter than the hour of mean solar time.

In even earlier systems of timekeeping, an hour was 1/12 of a period of daylight or darkness—hence, variable in length with seasonal changes in the length of day and night. The custom of dividing the cycle of day and night into 24 periods seems to have originated with the ancient Egyptians.

At the heart of the solar system is our sun. The four planets nearest it are rocky, terrestrial worlds — Mercury, Venus, Earth and Mars. After that are four gas giants — Jupiter, Saturn, Uranus and Neptune. Between the orbits of Mars and Jupiter lies the asteroid belt, which includes the dwarf planet Ceres. Beyond the orbit of Neptune one finds the disk-shaped Kuiper belt, in which dwarf planet Pluto resides, and far beyond that is the giant, spherical Oort Cloud and the teardrop-shaped heliopause. See the structure of the solar system (infographic).

Uranus is the third largest planet in the solar system, after Jupiter and Saturn. Uranus is about four times bigger than Earth. Its diameter at the equator is about 31,763 miles (51,118 kilometers), as measured at the level of the atmosphere where the pressure is the same as at sea level on Earth. The planet is slightly larger than Neptune, but Neptune is about 1.2 times more massive. Uranus’ density is quite low—only about 1.3 times that of water, compared with 1.6 for Neptune and 5.5 for rocky Earth.

The planet that Voyager uncovered is a stormy, windswept world with a vivid blue hue. Its highly active atmosphere is surprising, since it receives so little sunlight to power its weather systems. Like the other giant outer planets, Neptune has no solid surface. It also has a system of rings and more than a dozen moons.

Neptune is the smallest of the four giant outer planets. The diameter at its equator is about 30,775 miles (49,528 kilometers), as measured at a level of the atmosphere where the pressure is 1 bar (the pressure at sea level on Earth). This makes it slightly smaller than Uranus but nearly four times as big as Earth. Neptune’s mass is about 1.2 times greater than Uranus’, however, and more than 17 times greater than Earth’s. It is the third most massive planet in the solar system, after Jupiter and Saturn. Like the other outer planets, Neptune has a low density—only about 1.6 times the density of water. However, it is the densest of the four, being roughly 25 percent denser than Uranus.

What also is amazing is the sheer size difference of planets. While humans think of Earth as a large planet, in reality it is dwarfed by the massive gas giants lurking at the outer edges of our Solar System. This article explores the planets in order of size, with a bit of context as to how they got that way.

No human was around 4.5 billion years ago when the Solar System was formed, so what we know about its birth comes from several sources: examining rocks on Earth and other places, looking at other solar systems in formation and doing computer models, among other methods. As more information comes in, some of our theories of the Solar System must change to suit the new evidence.

The ocean is salty for several reasons, one of which is the groundwater from rain which trickles down into the ocean and picks up salt from the earth along its way! But this same water trickles down into lakes… so why aren't lakes salty?

The reason lake water usually isn't salty (or at least it's less salty than ocean water) is because the salty groundwater not only runs into the lakes, but also back out of it! Unlike the huge, vast ocean which already covers most of the Earth and therefore can't open up to any larger body of water, most lakes have outlets from which water leaves. This means that the salty groundwater has a chance to filter out and escape! (In the ocean, it has nowhere e

lse to go!)

Jupiter is the largest planet in the solar system. It is approximately 143,000 kilometers (about 89,000 miles) wide at its equator. Jupiter is so large that all of the other planets in the solar system could fit inside it. More than 1,000 Earths would fit inside Jupiter.

As the Earth rotates it turns away from the sun’s light, and the sky becomes dark. However, if you think about the millions and millions of bright stars all around the sky at night, you might start to wonder why it isn’t just as bright as it was when we were facing the sun!

If you weigh 70 pounds (32 kg) on Earth, on Jupiter you would weigh 185 pounds (84 kg). This is because Jupiter is such a large planet and so has more gravity.

A day consists of two 12-hour periods, for a total of 24 hours. An hour consists of 60 minutes. A minute consists of 60 seconds. Seconds are subdivided on a decimal system into things like "hundredths of a second" or "millionths of a second."

hat's a pretty bizarre way to divide a day up. We divide it in half, then divide the halves by twelfths, then divide the twelfths into sixtieths, then divide by 60 again, and then convert to a decimal system for the smallest increments. It's no wonder children have trouble learning how to tell time.

How long is a day? It's the amount of time it takes for the Earth to rotate one time on its axis. But how long does it take the Earth to rotate? That is where things become completely arbitrary. The world has decided to standardize on the following increments:

Although Jupiter’s orbit, and therefore its year, is so much longer than the Earth’s, its day is much shorter.  The Earth turns on its own axis, turning away from the Sun and so giving us day and night, once every 24 hours.  Jupiter spins round much faster, turning on its axis once every 9.84 hours!

Jupiter has a very faint ring system, but is totally invisible from the Earth. (The rings were discovered in 1979 by Voyager 1.) The atmosphere is very deep, perhaps comprising the whole planet, and is somewhat like the Sun. It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. At great depths within Jupiter, the pressure is so great that the hydrogen atoms are broken up and the electrons are freed so that the resulting atoms consist of bare protons. This produces a state in which the hydrogen becomes metallic.

Jupiter's Great Red Spot was discovered in 1664 by Robert Hooke. Part of the reason it lasts so long in comparison with Earth storms (other than the huge difference in scale and the existence of an internal heat source on Jupiter) is that this storm never goes over land; on Earth, hurricanes loses most of their energy and die when they stray over land.

The ocean is also salty from rainwater that falls onto rocks or land, picks up some salt in the earth, and then trickles down into the ocean! There are also some places in the ocean floor where salty gases and minerals from within the Earth get released into the ocean!  

The planet is one of the brightest objects in the night sky, and even a small telescope can reveal its multicolored stripes. These stripes are bands of clouds being pushed around the planet by strong east-west winds. Jupiter is a world of complex weather patterns. Its most prominent feature is an orange-red oval called the Great Red Spot. The oval is a storm system that has lasted at least 300 years and is bigger across than Earth and Mars combined.