Uranus’ distance from the Sun is 2.88 billion km. The exact number is 2,876,679,082 km. Want that number in miles? Uranus’ distance from the Sun is 1.79 billion miles.
This number is just an average, though. Uranus follows an elliptical orbit around the Sun. At its closest point, called perihelion, Uranus gets to within 2.75 billion km of the Sun. And then at its most distant point, called aphelion, Uranus gets to within 3 billion km from the Sun.
Astronomers use another term called “astronomical units” to measure distance within the Solar System. 1 astronomical unit, or AU, is the average distance from the Earth to the Sun – about 150 million km. So in astronomical units, Uranus is an average distance of 19.2 AU. Its perihelion is 18.4 AU, and its aphelion is 20.1 AU.
Saturn’s distance from the Sun is 1.4 billion km. The exact number for Saturn’s average distance from the Sun is 1,433,449,370 km.
Need that number in miles? Saturn’s average distance from the Sun is 891 million miles.
Noticed that I said that these numbers are Saturn’s average distance from the Sun. That’s because Saturn is actually following an elliptical orbit around the Sun. Some times it gets closer, and other times it gets more distant from the Sun. When it’s at the closest point of its orbit, astronomers call this perihelion. At this point, Saturn is only 1.35 billion km from the Sun. Its most distant point in orbit is called aphelion. At this point, it gets out to 1.51 billion km from the Sun.
Astronomers use another measurement tool for calculating distance in the Solar System called “astronomical units”. 1 astronomical unit is the average distance from the Earth to the Sun; approximately 150 million km. At its closest point, Saturn is 9 AU, and then at its most distant point, it’s 10.1 AU. Saturn’s average distance from the Sun is 9.6 AU.
Habitability in our solar system. Credit: UPR Arecibo, NASA PhotoJournal
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If humans were forced to vacate Earth, where is the next best place in our solar system for us to live? A study by the University of Puerto Rico at Arecibo has provided a quantitative evaluation of habitability to identify the potential habitats in our solar system. Professor Abel Mendez, who produced the study also looked at how the habitability of Earth has changed in the past, finding that some periods were even better than today.
Mendez developed a Quantitative Habitability Theory to assess the current state of terrestrial habitability and to establish a baseline for relevant comparisons with past or future climate scenarios and other planetary bodies including extrasolar planets.
“It is surprising that there is no agreement on a quantitative definition of habitability,” said Mendez, a biophysicist. “There are well-established measures of habitability in ecology since the 1970s, but only a few recent studies have proposed better alternatives for the astrobiology field, which is more oriented to microbial life. However, none of the existing alternatives from the fields of ecology to astrobiology has demonstrated a practical approach at planetary scales.”
His theory is based on two biophysical parameters: the habitability (H), as a relative measure of the potential for life of an environment, or habitat quality, and the habitation (M), as a relative measure of biodensity, or occupancy. Within the parameters are physiological and environmental variables which can be used to make predictions about the distribution, and abundance of potential food (both plant and microbial life), environment and weather.
The image above shows a comparison of the potential habitable space available on Earth, Mars, Europa, Titan, and Enceladus. The green spheres represent the global volume with the right physical environment for most terrestrial microorganisms. On Earth, the biosphere includes parts of the atmosphere, oceans, and subsurface (here’s a biosphere definition). The potential global habitats of the other planetary bodies are deep below their surface.
Enceladus has the smallest volume but the highest habitat-planet size ratio followed by Europa. Surprisingly, Enceladus also has the highest mean habitability in the Solar System, even though it is farther from the sun, and Earth, making it harder to get to. Mendez said Mars and Europa would be the best compromise between potential for life and accessibility. n Oct. 5, 2008. Image credit: NASA/JPL/Space Science Institute Cassini came within 25 kilometers (15.6 miles) of the surface of Enceladus o
“Various planetary models were used to calculate and compare the habitability of Mars, Venus, Europa, Titan, and Enceladus,” Mendez said. “Interestingly, Enceladus resulted as the object with the highest subsurface habitability in the solar system, but too deep for direct exploration. Mars and Europa resulted as the best compromise between habitability and accessibility. In addition, it is also possible to evaluate the global habitability of any detected terrestrial-sized extrasolar planet in the future. Further studies will expand the habitability definition to include other environmental variables such as light, carbon dioxide, oxygen, and nutrients concentrations. This will help expand the models, especially at local scales, and thus improve its application in assessing habitable zones on Earth and beyond.”
Studies about the effects of climate change on life are interesting when applied to Earth itself. “The biophysical quantity Standard Primary Habitability (SPH) was defined as a base for comparison of the global surface habitability for primary producers,” Mendez said. “The SPH is always an upper limit for the habitability of a planet but other factors can contribute to lower its value. The current SPH of our planet is close to 0.7, but it has been up to 0.9 during various paleoclimates, such as during the late Cretaceous period when the dinosaurs went extinct. I’m now working on how the SPH could change under global warming.”
The search for habitable environments in the universe is one of the priorities of the NASA Astrobiology Institute and other international organizations. Mendez’s studies also focus on the search for life in the solar system, as well as extrasolar planets.
“This work is important because it provides a quantitative measure for comparing habitability,” said NASA planetary scientists Chris McKay. “It provides an objective way to compare different climate and planetary systems.”
“I was pleased to see Enceladus come out the winner,” McKay said. “I’ve thought for some time that it was the most interesting world for astrobiology in the solar system.”
Mendez presented his results at the Division for Planetary Sciences of the American Astronomical Society meeting earlier this month.
Art by Sean McNaughton, National Geographics Staff; Sameul Velasco, 5@ infographics. Sources: NASA; Chris Gamble. Sund, asteroid and comet images: NASA/JPL
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National Geographic has put together a really nice zoomable poster on the history of robotic space exploration. It looks a little psychedelic from a distance, but zoom right in and follow the different missions to the various locations in our solar system, and see where the missions currently underway — like New Horizons, on its way to Pluto, and the venerable Voyagers that we hear from occasionally– are presently located. Click on the image to go to National Geographic’s Map of the Day page. Enjoy!
Plasma on the surface of the Sun. Image credit: Hinode
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The Sun is the hottest place in the Solar System. The surface of the Sun is a mere 5,800 Kelvin, but down at the core of the Sun, the temperatures reach 15 million Kelvin. What’s going on, why is the Sun hot?
The Sun is just a big plasma ball of hydrogen, held together by the mutual gravity of all its mass. This enormous mass pulls inward, trying to compress the Sun down. It’s the same reason why the Earth and the rest of the planets are spheres. As the pull of gravity compresses the gas inside the Sun together, it increases the temperature and pressure in the core.
If you could travel down into the Sun, you’d reach a point where the pressure and temperature are enough that nuclear fusion is able to take place. This is the process where protons are merged together into atoms of helium. It can only happen in hot temperatures, and under incredible pressures. But the process of fusion gives off more energy than it uses. So once it gets going, each fusion reaction gives off gamma radiation. It’s the radiation pressure of this light created in the core of the Sun that actually stops it from compressing any more.
The Sun is actually in perfect balance. Gravity is trying to squeeze it together into a little ball, but this creates the right conditions for fusion. The fusion releases radiation, and it’s this radiation that pushes back against the gravity, keeping the Sun as a sphere.
An astronomical unit is a method that astronomers use to measure large distances in the Solar System. 1 astronomical unit, or 1 au, is the average distance from the Sun to the Earth.
The Earth’s orbit around the Sun is actually elliptical. It varies from 147 million km to 152 million km. So the measurement of an astronomical unit is just the Earth’s average distance from the Sun. That’s where the more precise measurement of 1 AU to KM (149,598,000 km) comes from.
Here are some other distances in the Solar System:
Mercury: 0.39 AU
Venus: 0.72 AU
Mars: 1.5 AU
Jupiter: 5.2 AU
Saturn: 9.6 AU
Uranus: 19.2 AU
Neptune: 30.1 AU
Pluto: 39.5 AU
Eris: 67.7 AU
Oort Cloud: 50,000 AU
Alpha Centauri: 275,000 AU
We have written many articles about large distances in space. Here’s an article that explains how far space is, and here’s an article about the distance to stars.
Zodiacal light can be seen in the sky before sunrise or after sunset. Credit: Yuri Beletsky/ESO Paranal
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Imagine you could see the position of the Sun, in the sky, relative to the stars (and galaxies, and quasars, and …). If you could, and if you plotted that position throughout the year you’d get a line; that line is called the ecliptic.
And why is it called the ecliptic? Because when the new or full Moon is very close to this, there will be an eclipse (of the Sun, and Moon, respectively).
The Earth goes round the Sun, in an orbit. That orbit defines a plane, which is an infinite two-dimensional sheet; the plane of the ecliptic.
The other planets in the solar system orbit the Sun in planes too, but those planes are slightly tilted with respect to the plane of the ecliptic … so transits of Venus (across the Sun) are quite rare (most times Venus passes either above or below the Sun, when it’s between Earth and the Sun). Mutual transits and occultations of planets are even rarer.
If you’re in a location relatively free of light pollution, on a clear, moonless night you may see zodiacal light. If you trace a line through the middle of it, you’re tracing the ecliptic (zodiacal light is due to reflection of sunlight off dust; dust in the solar system is concentrated in a plane close to the ecliptic plane).
Today astronomers use equatorial coordinates to give positions on the sky, right ascension (RA) and declination (Dec); these are like projections of longitude and latitude out into space (or onto the celestial sphere). However, in Europe ecliptic coordinates were used (up to the 17th century anyway). Here’s a curious fact: historically, Chinese astronomers used equatorial coordinates!
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Want to find some cool Solar System coloring pages? Here are some links to resources we’ve been able to dig up from around the Internet.
Check out the offerings from Coloring Castle. I find it cool that they offer a version with Pluto, and then another without Pluto.
And one of the best resources on the internet for this kind of thing is Enchanted Learning. They’ve got a page just for Solar System coloring pages.
Windows on the Universe has coloring pages for all the planets in the Solar System. They even have an entire PDF book that you can print off with all the planets (including Pluto).
Coloring Fun has some more solar system pages for coloring.
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“The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit” That’s Kepler’s third law. In other words, if you square the ‘year’ of each planet, and divide it by the cube of its distance to the Sun, you get the same number, for all planets.
(The other two are “the orbit of each planet is an ellipse with the Sun at a focus”, and “a line between a planet and the Sun sweeps out equal areas in equal times”.)
Copernicus, Kepler, and Newton dealt a one-two-three knockout blow to the idea – thousands of years old – that the Sun (and planets) moved around the Earth. Copernicus put the Sun at the center, Kepler modified Copernicus’ circular motions (and provided a simple, quantitative description of the actual motion), and Newton explained how it all worked (gravity).
Kepler worked out his three laws from detailed records of observations of the positions of the planets (known at the time, Mercury, Venus, Mars, Jupiter, and Saturn) – especially Mars – painstakingly compiled by Tycho Brahe.
Kepler’s third law (in fact, all three) works not only for the planets in our solar system, but also for the moons of all planets, dwarf planets and asteroids, satellites going round the Earth, etc. Well, not quite; if the secondary body – a planet, say – has a mass that’s a significant fraction of the primary one (the Sun, say), then the law needs a small tweak.
By showing how Kepler’s laws could be derived from his universal law of gravitation, Newton united heaven and earth, perhaps the greatest revolution in science (OK, Darwin’s revolution may be greater). Before Newton, the heavens were thought to work according to rules quite different from the ones which governed things on Earth.
NASA’s Imagine the Universe! has a neat demonstration of Kepler’s laws, and this PDF file (from the University of Tennessee Knoxville’s Maths Department) gives a simple derivation of Kepler’s laws, from Newton’s universal law of gravitation.
The Earth’s circumference – the distance around the equator – is 40,075 kilometers around. That’s sounded nice and simple, but the question is actually more complicated than that. The circumference changes depending on where you measure it. The Earth’s meridional circumference is 40,008 km, and its average circumference is 40,041 km.
Why are there different numbers for the Earth’s circumference? It happens because the Earth is spinning. Think about what happens when you spin around holding a ball on a string. Your rotation creates a force that holds the ball out on the end of the string. And if the string broke, the ball would fly away. Even though the Earth is a solid ball of rock and metal, its rotation causes it to flatten out slightly, bulging at the equator.
That bulge isn’t very much, but when you subtract the meridional circumference (the equator when you pass through both poles), and the equatorial circumference, you see that it’s a difference of 67 km. In other words, if you drove your car around the equator of the Earth, you would drive an extra 67 km than you would if you drove from pole to pole to pole.
And that’s why the average circumference of Earth is 40,041 km. Which answer is correct? It depends on how accurate you want to be with your calculation.
