Posted on by Jean Tate

Parabolic Mirror

Herschel in 3-D. Credit: Nathanial Burton-Bradford.

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Sometimes, in astronomy, the name of a thing describes it well; a parabolic mirror is, indeed, a mirror which has the shape of a parabola (an example of a name that does not describe itself well? How about Mare Nectaris, “Sea of Nectar”!). Actually, it’s a circular paraboloid, the 3D shape you get by rotating a parabola (which is 2D) around its axis.

The main part of the standard astronomical reflecting telescope – the primary mirror – is a parabolic mirror. So too is the dish of most radio telescopes, from the Lovell telescope at Jodrell Bank, to the telescopes in the Very Large Array; note that the dish in the Arecibo Observatory is not a parabolic mirror (it’s a spherical one). Focusing x-ray telescopes, such as Chandra and XMM-Newton, also use nested parabolic mirrors … followed by nested hyperbolic mirrors.

Why a parabolic shape? Because mirrors of this shape reflect the light (UV, IR, microwaves, radio) from distant objects onto a point, the focus of the parabola. This was known in ancient Greece, but the first telescope to incorporate a parabolic mirror wasn’t made until 1673 (by Robert Hooke, based on a design by James Gregory; the reflecting telescope Newton built used a spherical mirror). Parabolic mirrors do not suffer from spherical aberration (spherical mirrors cannot focus all incoming, on-axis, light onto a point), nor chromatic aberration (single lens refracting telescopes focus light of different colors at different points), so are the best kind of primary mirror for a simple telescope (however, off-axis sources will suffer from coma).

The Metropolitan State College of Denver has a cool animation of how a parabolic mirror focuses a plane wave train onto a point (the focus).

Universe Today has many articles on the use of parabolic mirrors in telescopes; for example Kid’s Telescope, Cassegrain Telescope, Where Did the Modern Telescope Come From?, Nano-Engineered Liquid Mirror Telescopes, A Pristine View of the Universe … from the Moon, Largest Mirror in Space Under Development, and 8.4 Metre Mirror Installed on Huge Binoculars.

Telescopes, the Next Level is an excellent Astronomy Cast episode, containing material on parabolic mirrors.

Posted on by Fraser Cain

How Far is Jupiter from the Sun?

Jupiter's Red Spot

The distance from the Sun to Jupiter is approximately 779 million km, or 484 million miles. The exact number is 778,547,200 km.

This number is an average because Jupiter and the rest of the Solar System follows an elliptical orbit around the Sun. Sometimes it’s closer than 779 million km, and other times it’s more distant. When Jupiter is at its closest point in its orbit, astronomers call this perihelion; for Jupiter, this is 741 million km. At its most distant point, called aphelion, Jupiter gets out to 817 million km.

Astronomers use the term “astronomical unit” as another method for measuring distances in the Solar System. An astronomical unit, or AU, is the average distance from the Sun to the Earth – 150 million km. Jupiter’s average distance from the Sun is 5.2 AU. Its closest point is 4.95 AU, and its most distant point is 5.46 AU.

We have written many articles about Jupiter for Universe Today. Here’s an article about how Jupiter might be able to wreck the Solar System, and here’s an article about Jupiter’s Great Red Spot.

If you’d like more info on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an entire episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Posted on by Fraser Cain

How Far is Neptune’s from the Sun?

Neptune

Neptune’s distance from the Sun is 4.5 billion km; more specifically, it’s 4,503,443,661 km. If you’re still using the Imperial system, that’s the same as 2.8 billion miles.

But this number is actually an average. Like all of the planets in the Solar System, Neptune follows an elliptical orbit around the Sun, so it’s sometimes closer and sometimes further than this average number. When Neptune is at its closest point to the Sun, called perihelion, it’s 4.45 billion km from the Sun. And then when it’s at its most distant point from the Sun, called aphelion, it’s 4.55 billion km from the Sun.

Astronomers also measure distance in the Solar System using a measuring tool called the “astronomical unit”. 1 astronomical unit, or AU, is the average distance from the Earth to the Sun; that’s about 150 million km. So, Neptune’s average distance from the Sun is 30.1 AU. Its perihelion is 29.8 AU, and it’s aphelion is 30.4 AU.

We have written many articles about Neptune for Universe Today. Here’s an article about Neptune’s moons, and here’s an article about how Neptune’s southern pole is the warmest place on the planet.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We’ve also recorded an episode of Astronomy Cast all about Neptune. Listen here, Episode 63: Neptune.

Posted on by Fraser Cain

Pluto’s Distance from the Sun

The Pluto system seen from the surface of Hydra. Credit: NASA

Pluto’s distance from the Sun is 5.9 billion km – the exact number is 5,906,376,272 km. Need that figure in miles? Pluto’s distance from the Sun is 3.67 billion miles.

Keep in mind that this distance is an average. Pluto follows a highly elliptical orbit around the Sun. At the closest point of its orbit, called perihelion, Pluto gets to within 4.44 billion km from the Sun. And then at its most distant point of its orbit, called aphelion, Pluto gets to within 7.38 billion km of the Sun.

Astronomers use another term to measure distance in the Solar System called “astronomical units”. 1 astronomical unit, or AU, is the average distance from the Earth to the Sun – about 150 million km. Pluto’s perihelion is 29.7 AU, and its aphelion is 49.3 AU. Pluto’s average distance, or semi-major axis, is 39.5 AU.

We have written many articles about Pluto for Universe Today. Here’s an article about why Pluto isn’t a planet any more, and here’s an article about methane in Pluto’s atmosphere.

Want more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve recorded several episodes of Astronomy Cast about Pluto. Here’s one, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Fraser Cain

How Far is Uranus from the Sun?

Uranus, seen by Voyager 2. Image credit: NASA/JPL

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.

We have written many articles about Uranus for Universe Today. Here’s an article about how many rings Uranus has, and here are some interesting facts about Uranus.

If you’d like more information on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We’ve also recorded an entire episode of Astronomy Cast all about Uranus. Listen here, Episode 62: Uranus.

Posted on by Fraser Cain

How Far is Saturn from the Sun?

Saturn. Image credit: Hubble

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.

We have written many articles about Saturn for Universe Today. Here’s an article about how NASA’s Spitzer space telescope discovered a huge ring around Saturn, and here’s a cool movie of an aurora around Saturn.

If you want more information on Saturn, check out Hubblesite’s News Releases about Saturn. And here’s a link to the homepage of NASA’s Cassini spacecraft, which is orbiting Saturn.

We have also recorded an entire episode of Astronomy Cast just about Saturn. Listen here, Episode 59: Saturn.

Posted on by Fraser Cain

How Long Does it Take Uranus to Orbit the Sun?

Uranus, seen by Voyager 2. Image credit: NASA/JPL

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Uranus orbits the Sun much further than the Earth, and so it takes much longer to orbit the Sun. How much longer? Uranus takes 84.3 years to complete its orbit around the Sun. Uranus was only discovered in 1781 by Sir William Herschel. Since a year takes just over 83 Earth years, it completed its first orbit since discovery in 1865, and then its second in 1949. It’ll only complete its 3rd orbit around the Sun since its discovery in 2033.

Unlike most of the planets, which have slightly tilted orbits, Uranus is completely tilted over on its side. It kind of looks like it’s rolling its way around as it orbits the Sun. What this means is that one of Uranus’ hemispheres is completely in sunlight for half of its orbit, and then its other hemisphere is in sunlight for the rest of its orbit. Each pole gets 42 years of continual sunlight, followed by 42 years of continual darkness.

The orbit of Uranus is about the same length as the average life expectancy for a human being. In other words, if you were born on Uranus, you would only experience a single birthday, if you were lucky, after living for more than 84 Earth years. And nobody would experience two birthdays.

We have written many articles about Uranus for Universe Today. Here’s an article about how many rings Uranus has, and here’s an article about the atmosphere of Uranus.

If you’d like more information on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have also recorded an entire episode of Astronomy Cast just about Uranus. Listen here, Episode 62: Uranus.

Posted on by Fraser Cain

How Long Does it Take Mercury to Orbit the Sun?

Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry

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Mercury is the closest planet to the Sun, and so it’s the fastest to orbit the Sun. In fact, Mercury only takes 88 days to orbit the Sun. In other words, Mercury’s orbit only takes 24% as long as Earth’s orbit.

If you were born on Mercury, you would have celebrated 4 times as many birthdays as you do on Earth. In other words, if you’re 10 here on Earth, you’d be 40 in Mercury years. Now that’s a possible way to grow up more quickly.

Mercury orbits the Sun at an average distance of only 57.9 million km. Compare this with Earth’s average orbital distance of 150 million km.

Unlike the other planets in the Solar System, Mercury doesn’t really experience any seasons. This is because Mercury has no atmosphere to trap heat from the Sun. Whichever side of Mercury is currently facing the Sun experience temperatures of up to 700 Kelvin. And then the side of the planet that’s in the shade dips down to only 100 Kelvin; that’s well below freezing. Even though Mercury is close, you would experience incredibly cold temperatures if you lived on the surface.

The orbit of Mercury was actually a great puzzle to astronomers until the 20th century. They couldn’t explain why the point of Mercury’s furthest orbit of the Sun was slowly drifting at a rate of 43 arcseconds per century. But this strange motion was finally explained perfectly by predictions made by Albert Einstein with his Theory of Relativity.

We have written many articles about Mercury for Universe Today. Here’s an article about Mercury giving up more secrets to the MESSENGER spacecraft, and here’s a massive mosaic image of Mercury.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded an entire episode of Astronomy Cast just about Mercury. Listen here, Episode 49: Mercury.

Posted on by Fraser Cain

How Long Does it Take Pluto to Orbit the Sun?

Hubble image of Pluto and some of its moons, Charon, Nix and Hydra. Image Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team

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Because Pluto orbits much further from the Sun than Earth, it takes much longer to orbit the Sun. In fact, Pluto takes 248 years to orbit the Sun. That’s because Pluto orbits at an average distance of 5.9 billion km from the Sun, while Earth only orbits at 150 million km. In fact, it takes so long for Pluto to orbit that Sun, that the dwarf planet hasn’t even completed a third of an orbit from when it was discovered back in February 18th, 1930.

Pluto has a highly elliptical orbit. Its distance from the Sun varies from 4.4 billion km to 7.4 billion km. And during this orbital period, Pluto goes through a few interesting changes. You might be surprised to learn that Pluto has an atmosphere. When it’s at its closest point to the Sun, Pluto’s atmosphere evaporates from the surface and surrounding the dwarf planet. And then when it gets further away, the atmosphere freezes again, coating the surface in a thin layer.

Pluto was only discovered in 1930 by Clyde W. Tombaugh. Because it takes 248 years to orbit the Sun, Pluto won’t have completed a full orbit until the year 2178.

We have written many articles about Pluto for Universe Today. Here’s an article about why Pluto isn’t a planet any more, and here’s an article about methane discovered in Pluto’s atmosphere.

If you’d like more information on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded several episodes of Astronomy Cast just about Pluto. Listen here, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Fraser Cain

How Long Does it Take Neptune to Orbit the Sun

Neptune

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Neptune orbits much further away from the Sun than the Earth, so its orbit takes much longer. In fact, Neptune takes 164.79 years to orbit around the Sun. That’s almost 165 times longer than Earth takes to orbit the Sun.

Here’s an interesting fact. Neptune was only discovered on September 23, 1846. At the time this article was written (2009), that was only 163 years ago. In other words, since its discovery, Neptune has not even made a single orbit around the Sun.

On July 11, 2011, Neptune will have completed one full orbit around the Sun. Finally, Neptune will be 1 year old.

Just like Earth, Neptune’s axis is tilted away from the Sun’s axis. This means that it experiences seasons as it orbits the Sun. For half of its orbit, Neptune’s northern hemisphere is tilted towards the Sun, and then for the second half of its orbit, its southern hemisphere is tilted towards the Sun. This differential heating creates very powerful winds on Neptune. In fact, Neptune has the strongest sustained winds on the Solar System, with winds measured at 2100 km/hour.

We have written many articles about Neptune for Universe Today. Here’s an article about the atmosphere of Neptune. And here’s an article about who discovered Neptune.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have also recorded an entire episode of Astronomy Cast just about Neptune. Listen here, Episode 63: Neptune.