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What’s up? Night sky for May 2011

Stargazers looking at the sky

May will be a great month for planet watchers, with four bright planets visible to the east before dawn.

THIS WILL BE A FANTASTIC MONTH for planet watchers, with a series of attractive close groupings in the eastern morning sky. Mercury, Venus, Mars and Jupiter will be in the same part of the sky, and you’ll get the chance to watch their shifting positions as the month progresses.

Why do they appear to move around relative to each other? It’s because they’re on independent orbits about the Sun and travelling at different speeds. The Earth is moving around the Sun too, and our shifting perspective adds to the apparent sky motion. In fact, the word ‘planet’ comes from the Greek, and means ‘wandering star’.

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.

May 1

There’ll be a fantastic planetary get-together in this morning’s eastern sky. First, Jupiter and Mars will be just less than half a degree (roughly one Moon width) apart. Jupiter will be the brighter, whitish-coloured one on the right, with ruddy-coloured Mars on the left. Also present will be the crescent Moon … below and to the left of the planet Venus, and left of the planet Mercury, and above and to the left of the Jupiter-Mars pair. It’ll be a fantastic sight! Why not try taking a photo of it?

May 3

New Moon occurs today at 3:51pm Sydney time (06:51 Universal Time).

The Moon

The Moon is always a popular target for stargazers.

May 7

The planets Venus and Mercury will be side-by-side in this morning’s eastern sky, only 1.5 degrees apart (about three Moon widths).

May 8

Mercury, the innermost planet, will be at its greatest angular distance (27 degrees) from the Sun this morning.

May 11

It is First Quarter Moon today at 5:33am Sydney time (May 10, 20:33 Universal Time). The period around 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.

Also this evening, the Moon will appear close to Regulus, the brightest star in the constellation Leo. The amazing thing about Regulus is that, although to the naked eye it appears to be one star, in reality it is composed of four stars grouped into two pairs, all gravitationally bound to each other! This sort of thing is not too uncommon, as many other stars are members of double, triple or quadruple systems too.

May 12

Another planetary grouping in this morning’s eastern sky, with Venus only half a degree (one Moon width) to the right of Jupiter, and Mercury about three Moon widths above and to the left.

May 14

This evening the almost-full Moon will be perched about 7 degrees above the planet Saturn.

May 15

Tonight the Moon, just a smidge short of being full, will be only 1.5 degrees (about three Moon widths) above and to the right of the star Spica, the brightest star in the constellation Virgo. Spica is a blue giant star, 7.4 times as big as our Sun, and the 15th-brightest star in our night sky. Also today, the Moon will be at the closest point in its orbit around the Earth, called perigee. The distance between the two bodies will be 362,133 kilometres.

Diagram showing planets in the morning sky

Four planets are visible in the morning sky. This diagram shows the view on May 16.

May 16

Yet another arrangement of planets in our morning sky to the east. Venus will be about three Moon widths below and to the left of Mercury, about eight Moon widths below and to the right of Jupiter, and about six Moon widths above ruddy-coloured Mars.

May 17

Full Moon occurs today at 8:09pm Sydney time (11:09 Universal Time).

May 18

Tonight, look for the Moon about four degrees (eight Moon widths) below and to the left of Antares. Antares is a red supergiant star, the brightest star in the constellation Leo and the 16th-brightest star in our night sky. And get this—Antares is 800 times the diameter of our Sun, so you can see why they call it a supergiant!

May 22-31

Venus, Mars and Mercury will do a dance with each other in the morning sky over the final week of the month, in close proximity to one another. Have a look each morning and see how the arrangement has changed.

May 25

It is Last Quarter Moon today at 3:52am Sydney time (May 24, 18:52 Universal Time).

May 27

Today the Moon will reach the farthest point in its orbit, apogee, at a distance from Earth of 405,003 kilometres.

May 30-31

The crescent Moon will join the Mars, Venus, Mercury triplet in the morning sky.

And here’s Melbourne Planetarium‘s fabulous astronomer, Tanya Hill, to show us what the month’s sky will look like in motion:

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 / Andreas O. Jaunsen / IYA2009 / Galileoscope.

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What’s up? September’s night sky

The Moon and planets for September 2010

Venus and Mars are prominent in the western sky during September, and Jupiter is shining away in the eastern sky. Saturn is low in the western sky after sunset, and sinks lower and lower as the days go past. By the end of the month it will have dipped below the horizon and we’ll have to wait until later in the year for it to make its reappearance. Unfortunately, Mercury is lost in the glare of the Sun this month.

Sep 1

The Moon will be near the star cluster known as the Pleiades, in the constellation Taurus. You’ll have to be up early though, as this takes place in the early morning hours. By tomorrow morning (Sep 2), the Moon will have moved along a bit in its orbit and it will no longer appear next to the cluster.

The Pleiades star cluster

The Pleiades star cluster

The Pleiades is also known as the Seven Sisters, as people with average eyesight can usually make out seven of the stars with the unaided eye (from a reasonably dark location of course; not standing under a streetlight). Some people with really good eyesight can make out a few more.

In fact, the Pleiades has hundreds of stars, as shown by the beautiful image at right. You’ll also see that it seems to have a lot of wispy gas clouds too. Well, the gas and the stars are not actually connected, although they are in the same region of space. The stars are actually slowly passing through the gas clouds as a bunch, and we just happen to be living at the right time in history for us to see them together like this.

By the way, the next time you see a Subaru car drive past, take a look at the brand badge on the grill—you’ll see that it is a group of stars. Subaru is the Japanese name for the Pleiades!

SpaceInfo sky view for September 5, 2010

September 5, 2010: The Moon near Castor and Pollux as seen from the Southern Hemisphere (left) and Northern Hemisphere (right).

Sep 2

Tonight is Last Quarter Moon, which is halfway between Full Moon and New Moon.

Sep 5

Before sunrise, the Moon will seem to be sitting above (or below, for Northern Hemisphere stargazers) two reasonably bright stars. These are Castor and Pollux, the twin stars of the constellation Gemini.

Sep 8

Tonight is New Moon, which is halfway between Last Quarter and First Quarter.

Also tonight, the Moon will be at its closest to Earth for the current lunar orbit, being 357,191km away. The Moon travels in an elliptical orbit, so sometimes it is closer and sometimes it is further away. When it is at its closest, like tonight, we say it is at “perigee”. The furthest point is called “apogee”, and this month it will occur on the 21st.

Sep 9

For Southern Hemisphere stargazers, if you look out to the west after sunset, you’ll see the crescent Moon just near what looks to be a reasonably bright star. In fact, it’s the famous ringed planet Saturn. If you have a small telescope, or know someone who does, take a look—you should be able to see its rings slightly tilted, and you should also be able to make a few of its moons.

Spaceinfo sky view for September 11, 2010

September 11, 2010: the Moon will be near Venus and Mars.

Sep 11

Tonight, the thin crescent Moon will appear to hover right between Mars and Venus. It’ll be a really beautiful sight. And it’ll be easy to tell which planet is which—Venus is much brighter and a whitish colour; Mars is dimmer and a ruddy orange colour.

The star Spica (the brightest star in the constellation Virgo) will be nearby too.

At this time, the Moon will have just passed its “new Moon” phase—the opposite of full Moon—and will be heading toward first quarter on the 15th.

Sep 14

The Moon will appear close to the star Antares, the brightest star in the constellation Scorpius (often called Scorpio). Antares is a huge, red supergiant star. If you compare the colouring between Antares and the planet Mars, you’ll see that they are very similar. And that’s how Antares got its name—the ancient Greek name for Mars is Ares, and Antares means “rival of Mars”.

Sep 15

Tonight is First Quarter Moon, which is halfway between New Moon and Full Moon.

Sep 21

The Moon is at apogee today (see note for Sep 8), at a distance of 406,167km.

The planet Jupiter is at opposition. This means that the Sun and Jupiter are on exactly opposite sides of the Earth…the Sun one way, Jupiter exactly 180 degrees the other way.

The upshot of this is that as the Sun sinks below the horizon in the west at sunset, Jupiter rises over the horizon in the east. This means that the planet will be in the sky the whole night, from sunset through to tomorrow’s sunrise, giving you a full night to observe it.

The time of a planet’s opposition usually is very close to another milestone…it’s closest approach to Earth during that particular orbit. And when a planet is at its closest, it looks bigger through a telescope and therefore better studies can be made of it.

So putting the two together, opposition and closest approach, and you can see why astronomers look forward to these times to do their observations.

SpaceInfo sky view for September 23, 2010

September 23, 2010: the Moon will be near Jupiter

Sep 23

It’s Full Moon tonight! As the Sun goes down in the west, the Moon will rise over the eastern horizon. And you’ll see a bright looking “star” nearby—that’s not a star, it’s actually the planet Jupiter. If you have a medium sized pair of binoculars, or a small telescope, take a look at the giant planet—you should be able to see from one to four of its largest moons, and with a telescope you should be able to make out some of its atmospheric “bands”. Jupiter’s four largest moons are called the Galilean moons, after Galileo who first saw them just over 400 years ago.

It’s also the Equinox today—the Spring Equinox for those in the Southern Hemisphere, and the Autumn or Fall Equinox for those in the Northern Hemisphere.

The Equinox means that the Sun “crosses” the celestial equator, at this time of year going from north to south, heralding the coming of Spring in the Southern Hemisphere and Autumn or Fall in the Northern Hemisphere.

SpaceInfo sky view for September 29, 2010

September 29, 2010: the Moon will be near the Pleiades star cluster and the star Aldebaran.

In reality, the Sun isn’t moving—it’s the combination of the Earth’s tilt and the position of the Earth in its orbit that makes the Sun appear to move north and south in the sky during the course of the year.

Sep 29

Twenty-eight days since it was last there (see Sep 1), the Moon will be back near the Pleiades star cluster in the constellation Taurus. But it won’t be in exactly the same position—this time, it’ll be nestled between the Pleiades and red Aldebaran, the brightest star in Taurus.

If you have any questions or comments on the night sky, please use the feedback form below. Happy stargazing!

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A new way to weigh planets

Diagram showing the Sun, Earth and Jupiter orbiting a common barycentre

Measurements of signals from pulsars (purple lines) are affected by Earth's movement in its orbit around the Solar System's centre of mass, the barycentre. The barycentre moves too, due to the gravitational influence of the other planets. By working backwards from the pulsar signals, scientists can work out the gravitational pull of the planets, and from that deduce their masses.

  • Radio waves from pulsars affected by Solar System’s gravity
  • Adjusting the pulsar wave measurements gives gravity readings
  • From the gravity readings, the planet’s masses can be found

Astronomers from Australia, Germany, the UK, Canada and the USA have come up with a new way to weigh the planets in our Solar System, using radio signals from pulsars.

“This is first time anyone has weighed entire planetary systems—planets with their moons and rings,” said team leader Dr. David Champion of the Max-Planck-Institut fuer Radioastronomie in Bonn, Germany.

“And we’ve provided an independent check on previous results, which is great for planetary science.”

Measurements of planet masses made this new way could feed into data needed for future space missions.

Until now, astronomers have weighed planets by measuring the orbits of their moons or the trajectories of spacecraft flying past them. That’s because mass produces gravity, and a planet’s gravitational pull determines the orbit of anything that goes around it—both the size of the orbit and how long it takes to complete.

The new method is based on adjustments astronomers have to make to signals from pulsars…small spinning stars that deliver regular ‘blips’ of radio waves.

The Earth is travelling around the Sun, and this movement affects exactly when pulsar signals arrive here. To remove this effect, astronomers calculate when the pulses would have arrived at the Solar System’s exact centre of mass, or barycentre, around which all the planets orbit.

Because the arrangement of the planets around the Sun changes all the time, the barycentre moves around too.

To work out its position, astronomers use both a table (called an ephemeris) of where all the planets are at a given time, and the values for their masses that have already been measured.

If these figures are slightly wrong, and the position of the barycentre is slightly wrong, then a regular, repeating pattern of timing errors appears in the pulsar data.

“For instance, if the mass of Jupiter and its moons is wrong, we see a pattern of timing errors that repeats over 12 years, the time Jupiter takes to orbit the Sun,” said Dr Dick Manchester of CSIRO Astronomy and Space Science.

The CSIRO's Parkes radio telescope

The CSIRO's Parkes radio telescope made most of the pulsar signal measurements.

But if the mass of Jupiter and its moons is corrected, the timing errors disappear. This is the feedback process that the astronomers have used to determine the planets’ masses.

Better measurements of planet masses

Data from a set of four pulsars have been used to weigh Mercury, Venus, Mars, Jupiter and Saturn with their moons and rings.

Most of these data were recorded with CSIRO’s Parkes radio telescope in eastern Australia, with some contributed by the Arecibo telescope in Puerto Rico and the Effelsberg telescope in Germany.

The masses were consistent with those measured by spacecraft. The mass of the Jovian system (Jupiter and its moons)—0.0009547921 times the mass of the Sun—is significantly more accurate than the mass determined from the Pioneer and Voyager spacecraft, and consistent with, but less accurate than, the value from the Galileo spacecraft.

The new measurement technique is sensitive to a mass difference of 200,000 million million tonnes—just 0.003% of the mass of the Earth, and one ten-millionth of Jupiter’s mass.

“In the short term, spacecraft will continue to make the most accurate measurements for individual planets, but the pulsar technique will be the best for planets not being visited by spacecraft, and for measuring the combined masses of planets and their moons,” said CSIRO’s Dr George Hobbs, another member of the research team.

Repeating the measurements would improve the values even more. If astronomers observed a set of 20 pulsars over seven years they’d weigh Jupiter more accurately than spacecraft have. Doing the same for Saturn would take 13 years.

“Astronomers need this accurate timing because they’re using pulsars to hunt for gravitational waves predicted by Einstein’s general theory of relativity”, said Professor Michael Kramer, head of the ‘Fundamental Physics in Radio Astronomy’ research group at the Max-Planck-Institut fuer Radioastronomie.

“Finding these waves depends on spotting minute changes in the timing of pulsar signals, and so all other sources of timing error must be accounted for, including the [influences] of Solar System planets.”

Adapted from information issued by CSIRO / D. Champion, MPIfR.

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