Jupiter and the Moon last week

On Tuesday morning of last week (the 3rd) I woke up early, which is a habit I am trying to get back into so that I can recommence my running, which I try to do first thing in the morning before work. In a semi-awake state I was listening to BBC Radio 5’s breakfast show co-presenter Rachel Burden and someone else on the show between 6 and 7 (was it George Riley the sports person or someone else??) discussing how they had seen the Moon and Jupiter very bright in the sky on Monday evening.

The following morning during the same 6-7am period, the discussion was resumed with remarks being made about how much Jupiter had moved away from the Moon. I decided to tweet into the show and to Rachel saying that it was the Moon which has moved, not Jupiter. To cut a long story short, I ended up being on the show on the Thursday morning to explain what was moving and why. Here is a link to a recording of my 3-minute stint, where Rachel and Nicky Campbell (Rachel’s co-presenter) interview me.

The evening before my being on the show, one of the show’s researchers had asked me to send in a list of the five most interesting facts about Jupiter. They then put those out as a tweet during my interview. I chose the following five, would you have chosen the same ones?

The tweet from BBC Radio 5 with the five most interesting facts about Jupiter.

The list of the five facts

I will talk about each of these five facts separately in future blogs over the next several weeks. I will also blog in a little more detail about the NASA mission to Europa and the proposed ESA mission Juice; both of which intend to study this fascinating moon in more detail.

Being on the radio it is, of course, impossible to show diagrams about the motion of the Moon and Jupiter; and there really wasn’t enough time to explain it properly. So, I have decided to put together these slides to explain it in a little more detail.

Jupiter and the Moon during the first week of February 2015

Getting software to show you what is in the sky is easy, and although for PCs and Macs you may end up paying several tens of pounds, for tablet devices the software is much cheaper, with many reasonable ones being free. I use “Skysafari”, which is not free but is not too expensive either. It is made by Carina Software, who made the wonderful Voyager Software on Macs that I used for many many years in my classes.

Below is a screen capture using this software of the sky as seen from London at 20:00 on Thursday the 5th of February 2015. As you can see, I have done the screen capture with Jupiter just to the left (East) of the middle of the window (in the software you can use your finger to move around and look in different directions such as north or north-east if you wish). The Moon is at about 7 o’clock from Jupiter in direction, if you imagine a clock face.

This is a screen capture from an app I use on my iPad called “Skysafari” which can show what is in the sky at any location and at any time and date. There are lots of other similar apps available, but I like this one the most of the ones I’ve tried. This is the screen capture looking south for 20:00 on Thursday the 5th of February 2015 as seen from London.

Below I show a sequence of screen captures of Jupiter and the Moon (I have zoomed in on just enough to show the two) from Monday evening (the 2nd) to Friday evening (the 6th), all at 20:00 to show the motion of the Moon compared to Jupiter’s position.

Jupiter and the Moon as seen on Monday the 2nd of February at 20:00 from London

Jupiter and the Moon as seen on Tuesday the 3rd of February at 20:00 from London

Jupiter and the Moon as seen on Wednesday the 4th of February at 20:00 from London

Jupiter and the Moon as seen on Thursday the 5th of February at 20:00 from London

Jupiter and the Moon as seen on Friday the 6th of February at 20:00 from London

I think these screen captures make it quite easy to see that Jupiter is staying fixed in the same place relative to the stars during this sequence (which spans 5 nights), and it is the Moon which is moving. Why is this?

The Motion of the Moon in the sky

The reason it is the Moon which appears to move against the background of Jupiter and the stars is because the Moon is orbiting us; whereas Jupiter is orbiting the Sun and the stars are not orbiting the Sun but are, along with the Sun, in fact orbiting the centre of our Milky Way galaxy.

As the Moon takes roughly 30 days to orbit the Earth (see this blog for the more precise figure, and the difference between how long it takes to orbit the Earth – the “sidereal month” – and how long it is between two New Moons – the “synodic months”), then if we divide by 30 we get that the Moon moves in its orbit about the Earth each day/night. This figure is only approximate (but good enough for our purposes) because (a) a sidereal month is not exactly 30 days and (b) the Moon moves in an ellipse and not a circle about the Earth, and so changes its speed at different points in the orbit, so does not move the same amount each 24 hour period.

As the Moon is in diameter, corresponds to 24 times the diameter of the Moon. This is quite a lot, and so the motion of the Moon from night to night against the background planets and stars is very easily seen, as the sequence of diagrams above show.

In fact, Jupiter is also moving against the background stars, but it does so much more slowly. Jupiter takes about 12 years to orbit the Sun, and so each year it moves roughly 1/12th of a full circle. Along with the Sun, the Moon and the other planets, Jupiter moves through the zodiacal constellations during its travels, and so moves roughly into a new zodiacal constellation each year. At the moment it is in Cancer, but by this time next year it will be in Leo, the next constellation along the zodiac to the East (to the left of Cancer in the diagram above). You may be able to notice that Jupiter has moved relative to the background stars in a month or two, but certainly by next February, if you remember where it is now, you will see a difference.

Finally, although we refer to the stars as “the fixed stars”, they are not fixed. They, along with our Sun, are orbiting the centre of our Milky Way galaxy. Our Sun will take 250 million years to do this, stars closer to the centre of the Milky Way will take less time and stars further out from the centre will take longer. This leads to the positions of the stars relative to each other changing, but the change is very very slow, taking tens of thousands of years to be noticeable.

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An exoplanet with a ring system found

Last week I came across this interesting story on the BBC website – an exoplanet which has been found to have a large ring system – the first such discovery to date. Saturn, of course, has the most famous ring system of any of the planets in our Solar System. They were visible to Galileo in 1610 when he first looked at Saturn, but he was not able to discern them as rings, this was first done by Dutch astronomer Christian Huygens, who also discovered Saturn’s largest moon Titan.

However, in fact Jupiter, Uranus and Neptune (the other three “gas giants”) also have ring systems, although they are much fainter and less extensive than Saturn’s. Uranus’ ring system was discovered by SOFIA’s predecessor, the Kuiper Airborne Observatory, in 1977. Jupiter’s ring system was the next to be discovered, by the Voyager 1 space probe in 1979. Neptune’s ring system was discovered in 1984 using a ground-based telescope at La Silla Observatory (the European Southern Observatory), and were later imaged by Voyager 2 in 1989.

The ring system discovered about this exoplanet has been found by a survey known as SuperWASP, which is a ground-based survey looking for exoplanets using the transit method (which I discussed here). The exoplanet is orbiting a star by the name of J1407, which lies about 420 light years from Earth. This star had been found to have a peculiar light curve, not fitting the light curve one sees when a “normal planet” transits it. Further analysis by the authors of this work, led by Dr. Kenworthy at Leiden Observatory in the Netherlands, suggests that the complexity in the light curve is due to the transiting planet having a ring system. Here is a link to the abstract of the team’s paper on the arXiv preprint website.

This is the first ever discovery of a ring system about an exoplanet.

The ring system is measured to be about 200 times larger than that around Saturn. Such a ring system would appear to be so large that, if Saturn’s ring system (which is impressive in its own right) were replaced by this one then the rings would be clearly visible from Earth, as this artist’s sketch attempts to show.

An artist’s impression of how the ring system would look if Saturn had such large rings. It would be many times larger than the Moon in angular size, and easily visible from Earth.

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Cardiff Castle Astronomical (and Astrological) Statues

On the clock tower of Cardiff Castle is a wonderful collection of Astronomical (or astrological) figures. Here is a little photo gallery of the figures. I will do a separate blog about Cardiff Castle and its history in the near future. Although the castle dates back to Roman times, most of what one sees these days was built by the Third Marquess of Bute in the late 1800s, with the clock tower itself being built in 1868.

Mars and the Sun on the clock tower of Cardiff Castle.

The clock tower shows statues of figures representing the Sun, the Moon, and all 5 “naked eye” planets: Mercury, Venus, Mars, Jupiter and Saturn. In a separate blog I will show photographs of the sumptuous interior of the castle, including a room in the clock tower which has a star-painted ceiling and many astronomical motifs.

Cardiff Castle, the clock tower is in the distance

Mars and the Sun on the clock tower of Cardiff Castle.

The Sun

The Moon

Mercury

Venus

Mars

Jupiter

Saturn

Which is your favourite castle?

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The planets this winter (2012/13)

As the nights are now getting longer (for those of us in the Northern Hemisphere) and the clocks are about to go back making sunset earlier, I thought it was about time I blogged about the planets which will be visible this winter. Of the 5 naked eye planets, Mercury, Venus, Mars, Jupiter and Saturn, 4 of them are visible this winter. These are Mercury, Venus, Jupiter and Saturn. Mars, unfortunately, will not be visible at all this winter.

Here is a table showing the rise, transit and setting times of the Sun and the 5 naked eye planets (the transit time is the time at which the object crosses the local meridian, an imaginary line going from due North to due South across the sky).

The Planets in mid-October

Planet	Rise Time	Transit Time	Set Time	Constellation
The Sun	07:44	12:58	18:11	Virgo
Mercury	10:11	14:24	18:40	Libra
Venus	04:09	10:41	17:12	Leo
Mars	11:50	15:47	19:44	Ophiuchus
Jupiter	20:12	04:20	12:25	Taurus
Saturn	08:08	13:20	18:33	Virgo

The Planets in mid-January

Planet	Rise Time	Transit Time	Set Time	Constellation
The Sun	08:12	12:22	16:34	Sagittarius
Mercury	08:22	12:16	16:11	Sagittarius
Venus	07:12	11:07	15:02	Sagittarius
Mars	09:15	13:48	18:22	Capricorn
Jupiter	12:54	20:51	04:52	Taurus
Saturn	02:11	07:09	12:06	Libra

The Planets in mid-March

Planet	Rise Time	Transit Time	Set Time	Constellation
The Sun	06:27	12:22	18:18	Pisces
Mercury	05:45	11:08	16:31	Aquarius
Venus	06:32	12:12	17:53	Aquarius
Mars	06:46	12:50	18:56	Pisces
Jupiter	09:09	17:10	01:15	Taurus
Saturn	22:17	03:19	08:17	Libra

Mercury

Mercury, the closest planet to the Sun, is the most difficult to see. Because it is close to the Sun it can never be seen in the middle of the night, it is only visible near sunset or sunrise. Over the next 5 months there are two opportunities to see Mercury, the first one being in early December. As the figure below shows, Mercury will be visible in the morning sky before Sunrise in December, with early December being the best time to see it, as its elevation above the eastern horizon will be greatest, about 10 degrees. The Earth turns 15 degrees each hour, so 10 degrees represents about 40 minutes before the Sun rises.

This figure shows the position of the planet Mercury in the morning sky between mid-November and the end of December. The y-axis is the elevation above the horizon, the x-axis is azimuth, where due East is 90 degrees, and due South is 180 degrees. The azimuth angles in this figure show the planet will be visible in the South-East. As this figure shows, Mercury’s maximum elevation is in early December, when it is about 10 degrees. This means it will be visible some 40 minutes before Sunrise.

The next opportunity to see Mercury is in mid-February, in the evening sky. As the figure below shows, on 17th/18th of February, Mercury will be about 10 degrees above the horizon in the Western sky after sunset.

This figure shows the visibility of Mercury in the morning sky between the dates of early February and the end of February. The y-axis shows elevation in degrees above the horizon. The x-axis is azimuth. Due West is 270 degrees, and due South is 180 degrees. The azimuth angles in this figure show the planet will be visible in the South-West.

Venus

Venus is currently (mid-October) easily visible in the Eastern sky before sunrise. The figure below shows where to find it. You cannot really fail to find Venus when it is visible, it is by far the brightest “star like” object in the sky, outshining any of the stars (and even Jupiter) by a factor of at least two. At the moment, with Venus and Sirius visible in the same part of the sky before sunrise, one can see quite clearly how much brighter Venus is than the brightest star in the sky. Over the next few months it will get lower and lower before Dawn, until by the end of December it will disapppear and will not be visible again this winter.

Where to find Venus in the pre-dawn sky. It is to the East of the constellation Orion and Sirius, the brightest star. Seeing Venus and Sirius at the same time shows clearly how much brighter Venus is than Sirius.

Venus in the morning sky from mid-October to mid-December. Over the next 2 months it will sink lower and lower, and by the end of December will not be observable for the rest of the winter. The y-axis shows elevation above the horizon in degrees. The x-axis shows azimuth, with due East being 90 degrees and due South being 180 degrees. Venus is visible in the South-East sky before sunrise at the moment.

Mars

Mars is not visible over the next 5 months. It will next be at opposition in April 2014, so will begin to become visible towards the end of 2013.

Jupiter

Jupiter is currently in the constellation Taurus. It is currently easily visible high in the early morning sky before sunrise. The figure below shows where to expect to see it at 1am in mid-October. It is currently near Aldebaran, but outshines this red giant star by a factor of over 30 (the visual magnitude of Jupiter at the moment is -2.7 and of Aldebaran is +1). It is the brightest object in that part of the sky, with only Venus in the pre-dawn sky outshining it. If you wait until January, Jupiter will be visible in the evening sky rather than after midnight. A small telescope will reveal Jupiter’s 4 Galilean satellites, Io, Europa, Ganymede and Callisto.

An image of Jupiter taken by the Cassini space probe.

A map showing Jupiter in the sky at 1am as seen in mid-October as seen from Cardiff.

Saturn

Saturn is not currently visible, but will be as we move towards winter. By December it will be visible in the morning sky. It is currently in the constellation Libra, where it will remain for the next 2 years. By mid-January, Saturn will be quite visible in the early hours of the morning, but it will be even higher by mid-March. You might find it easier to wait until the Spring to see Saturn, as by April it will be quite high before midnight.

Saturn, the ringed planet. This image clearly shows the Cassini division in the rings.

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Humanity’s most distant travellers (part 1)

Next Thursday (27th of September), I am going to be on live TV talking about the Voyager space probes. These remarkable crafts left our small planet in 1977, and are now on the point of entering interstellar space. Voyager 1 is currently 18.2 billion km from the Sun, and Voyager 2 is 14.8 billion km away. They are, by a large margin, the most distant objects human beings have ever sent into space.

This cartoon, from the official NASA Voyager website, shows where the two spacecraft are, compared to the heliopause. The heliopause is a theoretical boundary where the Sun’s Solar wind is stopped by the stellar winds from other, nearby stars. The heliosphere is the sphere of space within which the Sun and the planets reside, as they are within the heliopause, the surface of this sphere (in reality it is not a sphere, it would only be a sphere if the radiation from nearby stars was perfectly uniformly incident on our Solar system).

This cartoon, from the NASA Voyager website, shows the position of the two Voyager space probes, and the heliosphere and heliopause.

Because these distances are so huge, it is sometimes easier to use larger units. The Astronomical Unit (AU) is defined as the average distance from the Earth to the Sun, so 150 million km. In these units, Voyager 1 is 122 AU from the Sun, and Voyager 2 is 99 AU away. The weak signals that we are still able to receive from the two space craft travel at the speed of light, and are currently taking about 30 hours (not minutes as I previously typed) to get to us.

The Voyager space probes were identical copies of each other, but were launched a few weeks apart and went on a different journey into the outer Solar System

Voyager 1 and 2 at Jupiter

Voyager 1 arrived at Jupiter in January 1979. Voyager 2 reached the planet in July of the same year. Both space craft returned the most detailed pictures yet of the Solar System’s largest planet. In addition to making important studies of the great red spot, the two probes made the surprising discovery of volcanic activity on Io, Jupiter’s closest moon.

A Voyager 1 image of Jupiter’s great red spot

The plume of material on the left is a volcanic eruption on the moon Io, the nearest of the Galilean moons.

We now know that Io is the most volcanically active body in the Solar System. The source of its internal heating is the tidal forces from Jupiter. Because it doesn’t orbit Jupiter in a perfect circle, but rather in an ellipse, the Moon gets repeatedly deformed in different directions and this heats its interior up (in the same way that squeezing a tennis ball repeatedly will lead to its getting warm). With a warm molten interior, the conditions are just right for this to escape through the crust as volcanic eruptions.

Voyager at Saturn

In November 1980, Voyager 1 flew past Saturn. By August 1981, Voyager 2 had arrived at the ringed planet. Voyager made important discoveries about Saturn and her moons, in particular about Saturn’s rings. It discovered new ring structures, and even “spokes” in the rings.

A backlit image of Saturn taken by Voyager 1 after its flyby in late 1980.

Voyager 2 discovered mysterious spokes in Saturn’s rings. It was many years before we understood what causes these.

Later this week I will write a part 2 to this blog, talking about Voyager 2’s encounters with Uranus and Neptune, the famous pale blue dot photograph, and the messages being carried on the probes as they head off into interstellar space.

Here are part 2 and part 3 of this post.

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The planets this summer (2012)

Back in November (2011), I wrote a blog on the planets which would be visible over the winter months. I thought it was about time, being over a week into the official summer, that I wrote a blog about the planets visible over the summer months this summer (2012). Unfortunately, there aren’t many planets visible this summer, Saturn and Mars is your lot.

This summer, Mars is to the Western side of the constellation Virgo, and is transiting at the moment (in early July) at 18:24. This means that, by the time it gets dark, which in Wales is not before 21:30 this time of year, Mars is quite far over to the West and on its way down in the sky. On the 3rd of March, Mars was at opposition, which means the Earth was at its closest to it. As a consequence, not only is Mars quite low (25 degrees above the horizon) by the time it gets dark, but it is also not very close to us. These two things combined mean Mars will be quite an unspectacular sight through a telescope.

Mars through a small telescope. If you are very lucky, you may see signs of the polar caps.

The other planet visible this summer is Saturn. Saturn is transiting at the moment (early July) at 19:55, so is reasonably high (30 degrees) in the sky after it has got dark. It is also to be found in the constellation Virgo, but over towards the constellation’s Eastern end, just to the North of the constellation’s brightest star Spica.

Saturn and Titan through a small telescope. Even with quite a small telescope, you should be able to see the rings and Titan quite easily.

Seeing Saturn for myself never ceases to excite me. Even through quite a small telescope one can clearly see the rings, and usually Saturn’s brightest moon Titan. If you want to see either Mars or Saturn this summer, then you really need to do so over the next few weeks, as by August they really will be setting too early to be able to see at all.

Although there aren’t too many planetary highlights this summer, there is still a lot to see in the Summer sky. One of the easiest things to find is the summer triangle, which is an asterism made up of Vega, Deneb and Altair (the brightest stars in the constellations Lyra, Cygnus and Acquila respectively).

The Summer Triangle, which is made up of the stars Vega, Deneb and Altair.

One of the other hightlights of the summer sky is the Ring Nebula, Messier 57. It is, in fact, what is called a Planetary Nebula. These are nothing to do with planets, but are in fact dying stars. Their name comes from the fact that, through 17th Century telescopes, they resembled the gas giant planets Jupiter and Saturn.

A planetary nebula is an object where the central star has thrown off its outer layers, and the remaining core (which we call a White Dwarf), is the remains of the once active star. The gases glow due to the electrons in the gas being excited by the energetic ultra violet light coming from the white dwarf. The white dwarf at the centre of the Ring Nebula is quite clearly visible through a medium-sized telescope.

The constellation Lyra (the harp), showing the location of Messier 57, the Ring Nebula

Messier 57, the Ring Nebula, one of the best planetary nebulae in the sky.

Our own Sun will end its life as a planetary nebula and white dwarf, as it is not massive enough to become e.g. a neutron star or a black hole. For a brief period (about 50,000 years), what hydrogen which the Sun will throw off during its asymptotic giant branch phase will glow in the sky, before fading from view as the white dwarf remains of the Sun slowly cools over time.

Update

I am going to be on BBC radio this Friday (13th of July 2012) talking about the summer sky. In preparing for this interview I realised that Jupiter is, of course, visible in the morning sky. It is to be found in the constellation Taurus, which is itself an easy constellation to find with the bright star Aldebaran in it. Jupiter is currently (mid July) rising at 02:45, so over the next few months is actually the best planet to see, by mid-August it will be rising about 00:45 and my mid-September by about 22:45.

Jupiter is in Taurus at the moment, just to the north of the bright red star Aldebaran, and to the East of Capella, “the Shepherd’s star”, which is in the constellation Auriga.

Jupiter is well worth looking at in a telescope. As I commented in my blog about the 2011/12 Winter sky, one can nearly always see the Galilean moons of Jupiter through a small telescope, and if one is lucky one can also see the bands and the great red spot. So, if you are out looking at the sky over the summer, don’t forget to stay up late (or get up early) to catch a glimpse of Jupiter.

Venus is in the same constellation. It is only some 5 weeks ago that Venus transited the Sun, but already it has moved to the West of the Sun in the sky so that it is now rising before it. Venus will appear as a large crescent at the moment, as it is on the near side to us in its orbit.

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