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Views of Moons

Jonathan Nally — Tue, 24 Apr 2012 14:00:02 +0000

NASA’S CASSINI SPACECRAFT, in orbit around the planet Saturn, has been sending back some wonderful views of its moons. In particular, it has captured images where one moon seems to float in front of the other. Here we present a selection of recent images.

Can you tell which of these moons in the foreground? It's Titan, the large one (diameter 5,150 kilometres; bigger than our Moon) with the orange atmosphere, with smaller, shiny, icy Tethys in the background. Titan was 2.3 million kilometres from Titan, and 3.4 million from Tethys when it took this image. Saturn's rings can be seen edge-on in the distance.

This black-and-white image shows the moon Rhea (1,528 km diameter) in front of Titan. Cassini was 2 million kilometres from Titan and 1.3 million kilometres from Rhea when it took this image.

This view shows Titan again, this time with the much smaller moon Dione (1,123 km diameter) peering around from behind, with Saturn and its rings (edge-on) in the background. Cassini was 2.3 million kilometres from Titan and 3.2 million kilometres from Dione when it took the image. The haze that surrounds Titan can clearly be seen. Titan has a mostly nitrogen atmosphere that extends far from the surface. The surface pressure is about 1.5 times that on Earth.

In this view, Titan appears to float in front of Saturn and its rings. Titan is not only the second-largest moon in the Solar System; it's also about 300 kilometres wider than the planet Mercury!

More information: Cassini mission

Story by Jonathan Nally. Images courtesy NASA / JPL-Caltech / Space Science Institute.

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What’s up? Night sky for April 2012

Jonathan Nally — Thu, 29 Mar 2012 14:00:51 +0000

The southern sky is full of wonderful treats for the stargazer. (This star trail photo by Iztok Boncina was made by keeping the camera shutter open.)

Except where indicated, all of the phenomena described here can be seen with the unaided eye. And unless otherwise specified, dates and times are for the Australian Eastern Standard Time zone, and sky directions are from the point of view of an observer in the Southern Hemisphere. (If you’re in the Northern Hemisphere, please see the video at the bottom of the page for What’s Up in your night sky.)

April 3

The Moon, now just over three-quarters full, will be about 11 Moon widths above and to the left of the bright blue star, Regulus, the brightest star of the constellation Leo. A little further below is what looks like a red star, but is actually the planet Mars. The colours of Regulus and Mars make a nice contrast. About 77.5 light years from Earth, Regulus is not one star but four, grouped into two pairs. Multiple star systems are very common throughout the Milky Way galaxy.

April 4

The Moon is still in the vicinity of Mars tonight, being above and to the right of the planet. Incidentally, when I use a term such as “vicinity”, it is not to be taken as suggesting the two bodies are physically near each other out there in space. Rather, they are simply within similar lines-of-sight from our vantage point on Earth.

April 4, 8:00pm. The Moon, Mars and the star Regulus will make an attractive triangle in the northern part of the sky. Note the colour difference between blue Regulus and ruddy Mars.

April 7

Full Moon occurs today at 5:19am Sydney time (19:19 Universal Time on April 6). In a similar fashion to its “encounters” with Regulus and Mars a few days earlier, tonight the Moon will be about six Moon widths to the right of another bright star, Spica, and about 10 Moon widths above the planet Saturn. Spica is the brightest star of the constellation Virgo; it is a blue giant star about 260 light years from Earth. This is a great time to see Saturn (see April 16), so it’s a good idea to use the nearby Moon to identify it.

April 8

Today the Moon will be at the closest point in its orbit, called perigee. The distance between the two bodies today will be 358,311 kilometres.

April 10

This evening, take a look about 12 Moon widths below the Moon and you’ll see a reddish looking star that looks a bit like Mars. It’s the star Antares, and its name actually “rival of Mars”. Antares is the brightest star of the constellation Scorpius, and is a red supergiant star about 883 times bigger than our Sun!

Saturn, as it appears through a backyard telescope. Image by Steve Massey.

April 13

It is Last Quarter Moon tonight at 8:50pm Sydney time (10:10 Universal Time).

April 16

Today the planet Saturn reaches what astronomers call “opposition”. This means that, from an Earthly perspective, it is the opposite direction to the Sun—so if you could look down on the Solar System from above you’d see the Sun, Earth and Saturn (in that order) in a straight line … although Saturn, of course, is much further from us than the Sun. The period around opposition is a considered a great time to view a planet, as it rises in the east around the same time as the Sun sets in the west, and is therefore nice and high in the sky by late evening.

April 19

If you’re an early riser, out to east this morning before sunrise you’ll see the very thin crescent Moon. Above and to its right is a bright looking star. Well that’s not a star; it’s the planet Mercury, the closest planet to the Sun.

April 19, 6:30am. The thin crescent Moon and the planet Mercury will be visible together in the eastern sky before sunrise.

April 21

New Moon occurs today at 5:18pm Sydney time (07:18 Universal Time).

April 23

Today the Moon will again reach the farthest point in its orbit, apogee, at a distance from Earth of 406,421 kilometres.

April 24

Today the Moon makes another apparent close approach to a star, this time Aldebaran, the brightest star of the constellation Taurus. The pair will be low in the western sky after sunset. Like Antares, Aldebaran too is a red star, but not a supergiant—it is only about 44 times the size of our Sun. It’s about 65 light years from Earth.

April 25

There’ll be a lovely astronomical pairing in this evening’s sky, with the Moon very close to Venus. The Moon is the second-brightest object in our night sky, and Venus is the third-brightest.

April 25, about 7:15pm. The thin crescent Moon and Venus will be close together in the western sky.

April 29

It is First Quarter Moon today at 7:58pm Sydney time (09:58 Universal Time). First Quarter is a good time to look at the Moon through a telescope, as the sunlight angle means the craters and mountains are throwing nice shadows, making it easier to get that 3D effect.

There’s more great night sky viewing information at Melbourne Planetarium’s Skynotes site.

If you’re in the Northern Hemisphere, here’s a Jet Propulsion Laboratory video that details what you can see this month:

If you have any questions or comments on the night sky, we’d be happy to answer them. Please use the Feedback Form below. Happy stargazing!

Images courtesy IAU and Iztok Boncina / ESO.

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New images of an icy world

Jonathan Nally — Tue, 13 Mar 2012 02:08:07 +0000

NASA's Cassini spacecraft took this raw, unprocessed image of Saturn's moon Rhea on March 10, 2012. The camera was pointing toward Rhea from a distance of approximately 41,873 kilometres.

THESE RAW, UNPROCESSED IMAGES of Saturn’s second largest moon, Rhea, were taken on March 10, 2012, by NASA’s Cassini spacecraft. This was a relatively distant flyby with a close-approach distance of 42,000 kilometres, well suited for global geologic mapping.

At 1,530 kilometres diameter, Rhea is the ninth-largest moon in the Solar System.

During the flyby, Cassini captured these views of the moon’s cratered surface, creating a 30-frame mosaic of Rhea’s leading hemisphere and the side of the moon that faces away from Saturn.

The observations included the large Mamaldi (480 kilometres across) and Tirawa (360 kilometres across) impact basins and the 47-kilometre-wide “ray crater”Inktomi, one of the youngest surface features on Rhea.

This second raw, unprocessed Cassini image of Rhea was taken from a distance of approximately 42,258 kilometres, and shows the moon's icy, cratered surface. The streaks on the right are an artefact of the imaging.

Shadows help to give a 3D effect to Rhea's craters in this raw, unprocessed Cassini shot taken from a distance of approximately 42,096 kilometres.

This raw, unprocessed shot was taken from much further away, approximately 115,060 kilometres, and shows Rhea's "terminator"—the dividing line between day and night.

Cassini has been investigating Saturn and its moons since 2004. This included dropping a probe called Huygens onto the surface of Saturn’s largest moon, Titan, in 2005. Launched in 1997, Cassini-Huygens mission is a co-operative project of NASA, the European Space Agency and the Italian Space Agency.

See all of Cassini’s raw images at NASA’s Saturn page.

Adapted from information issued by NASA / JPL-Caltech / SSI.

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What’s up? Night sky for January 2012

Jonathan Nally — Sun, 01 Jan 2012 04:19:26 +0000

Summer nights are perfect for stargazing.

Except where indicated, all of the phenomena described here can be seen with the unaided eye. And unless otherwise specified, dates and times are for the Australian Eastern Daylight Time (AEDT) zone, and sky directions are from the point of view of an observer in the Southern Hemisphere.

January 1

It is First Quarter Moon today at 5:15pm AEDT. First Quarter is a good time to look at the Moon through a telescope, as the sunlight angle means the craters and mountains throw nice shadows, making it easier to get that 3D effect.

January 3

Today the Moon will reach the farthest point in its orbit from the Earth, which is called apogee. It’s distance from our planet will be 404,578 kilometres.

While you’re out looking at the Moon, you’ll notice a bright looking star above and to the left. Well, that’s not a star; it’s actually the planet Jupiter. If you have access to a telescope, or even a pair of 7×50 (or better) binoculars, train them on Jupiter and you should see up to four tiny pinpoints of light on either or both sides of the planet—these are the moons Galileo discovered, Io, Europa, Ganymede and Callisto. If you take a look again tomorrow night, you’ll see that their positions will have changed as they whiz around the planet.

And in fact, we’re only a few days away from the anniversary of their discovery. On the 7th of this month, it will be 402 years since Galileo spotted them!

January 5

Today the Earth reaches perihelion in its orbit around the Sun. Perihelion is the point in a solar orbit when the body in question (eg. Earth) is at its closest to the Sun. Perihelion occurs today at midday AEDT, at a distance between Earth and Sun of about 147,098,000 kilometres. (The opposite of perihelion is aphelion, which for Earth will occur on July 5, 2012 at a distance of about 152,098,000 kilometres.)

If you have a pair of binoculars, you’ll see a pretty sight tonight, with the soon-to-be-full Moon appearing to sit above a beautiful star cluster called the Pleiades, or Seven Sisters. When the Moon is not around and the sky is dark, most people can make out 6 to 7 of the Pleiades stars, although eagle-eyed stargazers can see a few more. There are actually hundreds of stars in this beautiful “open star cluster“, and it is also filled with beautiful whispy gas clouds, although the stars and the gas are not actually related to each other—we just happen to be seeing them at a time when the stars are drifting through the gas.

This is the view in the evening of January 5, with the Moon sitting above the beautiful star cluster known as the Pleiades. A pair of binoculars will show the scene well. Tomorrow night the Moon will have shifted east, and will be near to Albebaran, the brightest star in the constellation Taurus.

January 6

Tonight you might notice a fairly bright, reddish-looking star just above the Moon. This is Aldebaran, the brightest star in the constellation Taurus.

January 9

Full Moon occurs today at 6:30pm AEDT.

January 12

Tonight, the Moon will appear above and to the right of a bright blue star. This is Regulus, the brightest star in the constellation Leo.

January 14

In this morning’s pre-dawn sky out to the east, the Moon will appear above and to the left of the planet Mars, which looks like a ruddy-coloured star. As you gaze at it, spare a thought for the Mars Science Laboratory, which was launched a little under two months ago and which is due to reach the Red Planet on August the 6th this year.

January 16

It is Last Quarter Moon today at 8:08pm AEDT.

January 17

In this morning’s sky, the Moon will be just above and to the right of the planet Saturn, which looks like a bright star. Nearby is Spica, the brightest star in the constellation Virgo.

The Moon will be near to both the star Spica and the planet Saturn (the bright yellowish "star" below the Moon) on January 17. If you have access to a small telescope, take a look at both the craters and mountains on the Moon and Saturn and its glorious rings.

January 18

Today the Moon will be at the closest point in its orbit, called perigee. The distance between the two bodies today will be 369,887 kilometres.

January 20

In the early dawn sky, take a look for the Moon and you should see a reddish-looking star just above it. This is Antares, the brightest star in the constellation Scorpius. Compare Antares with Mars—do you think they look similar? The ancients thought so, and in fact the name Antares means “rival of Mars”.

January 23

New Moon occurs today at 8:39pm AEDT.

January 27

Tonight, there’ll be a lovely sight in the evening sky out to the west, with the crescent Moon paired with the bright planet Venus (about 20 Moon widths to its left).

January 30

This evening, the Moon meets up with Jupiter again, appearing below the planet in the northwestern part of the sky.

Also today, the Moon will again reach apogee again, at a distance from Earth of 404,323 kilometres.

The Moon will sit just below the planet Jupiter in the evening sky of January 30.

January 31

And finally for January, we have a second First Quarter Moon, which occurs today at 3:10pm AEDT.

There’s more great night sky viewing information at Melbourne Planetarium’s Skynotes site.

If you have any questions or comments on the night sky, we’d be happy to answer them. Please use the Feedback Form below. Happy stargazing!

Main image courtesy IAU.

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What’s up? The night sky for December 2011

Jonathan Nally — Wed, 30 Nov 2011 01:00:52 +0000

Stargazing is great fun, now that the warmer summer weather is here.

December 2

It is First Quarter Moon today at 8:52pm Australian Eastern Daylight Time (AEDT). First Quarter is a good time to look at the Moon through a telescope, as the sunlight angle means the craters and mountains throw nice shadows, making it easier to get that 3D effect.

December 6

Today the Moon will reach the farthest point in its orbit, apogee, at a distance from Earth of 405,412 kilometres. And if you take a look at the Moon this evening, you’ll see a bright ‘star’ above and to its right. That’s not a star—it’s actually the planet Jupiter!

December 9

Take a look at the Moon in this evening’s sky, and you’ll see a brightish star a little way out to its right. And yes, this one really is a star. It’s called Aldebaran, and it’s the brightest star in the constellation Taurus. Aldebaran is a red giant star roughly 44 times as big as the Sun, and is about 65 light-years from Earth.

December 10

Full Moon occurs today at 1:36am AEDT, and tonight everyone in Australia and New Zealand will experience a total lunar eclipse. See our separate lunar eclipse story for full details on how, when and where to see it.

Eclipses aside…although it looks very pretty high up there in the sky, astronomers, both amateur and professional, generally hate the full Moon. This is because its light tends to drown out many of the fainter objects they’re interested in seeing. (It does this by actually making the sky glow.) It’s also not a good time to look at the Moon itself through a telescope, as the overhead sunlight (as seen from the perspective of the Moon) doesn’t throw any shadows across the lunar surface—and shadows are what give the craters and mountains their 3D look.

The Moon looks great through a telescope, but you won't need one to see the total lunar eclipse on December 10, 2011.

December 17

If you’re up early today, look for the Moon and you’ll see that it seems to have two companions. A little way below and to its left is the star Regulus, and below and to its right is the planet Mars. Regulus is actually a quadruple star system, comprised of four stars in two groups of two, gravitationally bound to one another. But the main star is a young, blue star a little over three times the mass of the Sun, and about three to four times as big as the Sun too.

December 18

It is Last Quarter Moon today at 11:48am AEDT. The Moon is still near Mars in the sky, being above and to the right of the planet in the hours before dawn.

December 21

The Moon, a star and a planet make a nice triangle in this morning’s sky. The star is Spica, the brightest star in the constellation Virgo, and the planet is Saturn. Saturn will be to the left of the Moon, and Spica will be above Saturn. Spica, a blue giant star, is the 15th brightest star in the night sky and is about 260 light-years from Earth.

December 22

There are two items of note for today. First, the Moon will be at the closest point in its orbit, called perigee, which is the opposite of apogee. The distance between the two bodies today will be 364,800 kilometres. And secondly, today marks the summer solstice in the Southern Hemisphere. This is the day of the year when the Sun is highest in the sky.

December 25

New Moon occurs today at 5:06am AEDT.

December 27-28

The Moon is back in the western evening sky. Over these two nights, it’ll be paired up with the planet Venus—the duo will make a very attractive sight in the evening dusk.

There’s more great night sky viewing information at Melbourne Planetarium’s Skynotes site.

If you have any questions or comments on the night sky, we’d be happy to answer them. Please use the Feedback Form below. Happy stargazing!

Story by Jonathan Nally. Images courtesy IAU.

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Gallery – Geyser moon seen in silhouette

Jonathan Nally — Mon, 28 Nov 2011 13:01:17 +0000

Saturn's moon Enceladus appears in silhouette in this image taken by NASA's Cassini spacecraft. The bulk of Saturn is in the background, the planet's rings seen edge-on appear as the dark horizontal line.

NASA’S CASSINI SPACECRAFT took this image of Saturn’s moon Enceladus on October 19, 2011. As the spacecraft passed Enceladus, its infrared instruments, cameras and other instruments monitored activity on the moon, in particular the famed jets erupting from the its south pole. The orbiter flew within about 1,230 kilometres of Enceladus’ surface.

Although it appears dark in the silhouetted view, Enceladus, 504 kilometres wide, is actually one of the most reflective bodies in the Solar System because it is constantly coated by fresh, white particles of ice.

Also visible are Saturn’s rings, seen edge on.

More information:

Enceladus – Saturn’s shiny moon

The eruptions of Enceladus

Adapted from information issued by NASA/JPL-Caltech/Space Science Institute.

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Gallery – Quintet of Saturnian moons

Jonathan Nally — Thu, 17 Nov 2011 13:01:03 +0000

Five of Saturn's moons can be seen in this image taken by NASA's Cassini spacecraft.

A QUINTET OF SATURN’S MOONS can be seen in this view taken by the Cassini spacecraft.

Janus (179 kilometres wide) is on the far left. Pandora (81 kilometres) orbits between the A ring and the thin F ring near the middle of the image. Brightly reflective Enceladus (504 kilometres) appears above the centre of the image. Saturn’s second largest moon, Rhea (1,528 kilometres), is bisected by the right edge of the image. The smaller moon Mimas (396 kilometres) can be seen beyond Rhea also on the right side of the image.

This view looks toward the northern, sunlit side of the rings from just above the ringplane. Rhea is closest to the camera. Saturn’s rings are beyond Rhea and Mimas. Enceladus also is beyond the rings.

The image was taken with the Cassini spacecraft narrow-angle camera on July 29, 2011. The view was acquired at a distance of approximately 1.1 million kilometres from Rhea and 1.8 million kilometres from Enceladus.

Adapted from information issued by NASA / JPL-Caltech / Space Science Institute.

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Gallery – Dione and friends

Jonathan Nally — Mon, 14 Nov 2011 13:01:00 +0000

Saturn's moon Dione, seen along with half of Saturn and the planets rings.

SATURN’S MOON DIONE coasts along in its orbit appearing in front of its parent planet in this Cassini spacecraft view.

The wispy terrain on the trailing hemisphere of Dione (1,123 kilometres wide) can be seen on the left of the moon here.

Dione (pronounced dy-OH-nee) is the second densest moon of Saturn, after Titan. Dione is probably composed of a rocky core making up one-third of the moon’s mass, and the rest is composed of water ice. It is similar to two other Saturnian moons, Tethys and Rhea.

Dione’s icy surface includes heavily cratered terrain, with moderately and lightly cratered plains, as well as some severely cracked areas, with very bright material on the walls of the fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres in diameter. The plains area tends to have craters less than 30 kilometres in diameter.

The moon Telesto can be seen above the rings on the left, and Epimetheus is just on the bottom edge of the rings.

The tiny moon Telesto (25 kilometres wide) is visible as a white speck above and to the left of the rings in this view. Epimetheus (113 kilometres) appears just below the rings near the centre of the image. This view looks toward the northern, sunlit side of the rings from just above the ringplane.

The image was taken with the Cassini spacecraft narrow-angle camera on July 18, 2011. The view was acquired at a distance of approximately 2.2 million kilometres from Dione.

Adapted from information issued by NASA / JPL-Caltech / Space Science Institute.

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Gallery – Saturn’s four moon shuffle

Jonathan Nally — Tue, 08 Nov 2011 14:01:12 +0000

Four of Saturn's moons are visible in the image taken by NASA's Cassini spacecraft.

A QUARTET OF SATURN’S MOONS, from tiny to huge, surround and are embedded within the planet’s rings in this Cassini image. Saturn itself is out of frame to the left.

Saturn’s largest moon, Titan (5,150 kilometres wide), is in the background of the image.

Next, in the foreground is Dione (1,123 kilometres wide), with the wispy terrain on its trailing hemisphere easily visible.

The third moon is Pandora(81 kilometres wide), which orbits just beyond the rings on the right of the image.

The tiny moon Pan appears as a speck in the gap in the rings.

Finally, tiny Pan (28 kilometres wide) can just be seen as a tiny speck in the ‘Encke Gap’ of the A ring on the left of the image.

Saturn has 62 known moons, with the vast majority of them being 50 kilometres or less in diameter.

The image was taken with the Cassini spacecraft narrow-angle camera on September 17, 2011, at a distance of approximately 2.1 million kilometres from Dione.

Adapted from information issued by NASA / JPL-Caltech / Space Science Institute.

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What’s up? Night sky for November 2011

Jonathan Nally — Sun, 06 Nov 2011 00:48:08 +0000

Late spring nights and mornings are ideal for stargazing, and there are some interesting things to see this month.

November 3

It is First Quarter Moon today at 3:38am Australian Eastern Daylight Time. First Quarter is a good time to look at the Moon through a telescope, as the sunlight angle means the craters and mountains are throwing nice shadows, making it easier to get that 3D effect.

November 9

Take a look in the evening sky and you’ll see the Moon with what looks like a bright star above and to its right. Well, that’s not a star, it’s the planet Jupiter. Also today, the Moon will reach the farthest point from Earth in its orbit, apogee, at a distance from Earth of 406,176 kilometres.

November 10

Take a look at the western horizon after sunset and you’ll see a pretty group comprising Venus, Mercury and the star Antares.

The Moon teams up with several planets during November

November 11

Full Moon occurs today at 7:16am Australian Eastern Daylight Time.

November 11-12

Out to the east in the early morning sky (pre-dawn) you’ll find a pair of celestial orbs that contrast each other nicely in colour. Ruddy coloured Mars will appear very close to Regulus, a blue giant star that is the brightest star in the constellation Leo.

November 19

It is Last Quarter Moon today at 2:09am Australian Eastern Daylight Time. If you’re up before dawn, take a look out to the eastern sky and you’ll see the Moon with the star Regulus close by, and the planet Mars about 4 degrees away as well.

November 23

Another attractive grouping, but quite low in the eastern sky before dawn (so you’ll need a clear horizon). There’ll be the Moon, plus the star Spica (the brightest star in the constellation Virgo) and the planet Saturn as well.

November 24

Today the Moon will be at the closest point to Earth in its orbit, called perigee, which is the opposite of apogee. The distance between the two bodies today will be 359,691 kilometres.

November 25

New Moon occurs today at 5:10pm Australian Eastern Daylight Time.

November 27

Take a look out to the west just after sunset, and you might see the very thin crescent Moon below and to the right of the planet Venus.

If you have any questions or comments on the night sky, we’d be happy to answer them. Please use the Feedback Form below. Happy stargazing!

Images courtesy IAU / TWAN / Babak /A. Tafreshi.

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