Posts Tagged ‘Planets’

A number of people have been asking me over the last two or three weeks what the (very) bright object is in the evening sky. It is Venus, the brightest of all the planets. If you look towards the west (the same part of the sky as where the Sun has set) on any clear(ish) evening over the next two months, within a few hours of sunset, you should easily be able to see Venus.

Here is a diagram showing the evening sky for this evening (12 January 2017) as seen from Cardiff, and I have set it up to show the sky at 6pm. In Cardiff today the Sun sets at 16:29. Venus will not set until 20:51, nearly 3.5 hours after the Sun has set. This is why it is visible for such a long time after sunset.


The western sky at 6pm as seen from Cardiff. Today the Sun will set in Cardiff at 4:29pm, with Venus not setting until 8:51pm. This is nearly 3.5 hours after sunset, and today is the day of maximum eastern elongation.

In fact, today (12 January) is the day when the time between the Sun setting and the time at which Venus sets is at its greatest. That is why I chose today to blog about Venus. This is called maximum eastern elongation, and it is shown in the diagram below.


When the angle between a line from Earth to Venus and Venus to the Sun is a right angle, we have maximum elongation. As Venus is currently to the East of the Sun (rising after and setting after the Sun), it is today at maximum eastern elongation.

Venus will dominate the evening sky for another 6 weeks or so, although it will start setting closer and closer to the time of sunset now that we have passed maximum eastern elongation. It will swing in front of the Sun (something called inferior conjunction) on 25 March, so will be lost in the glow of the Sun for a few weeks before that. A few weeks after inferior conjunction, it will reappear as a morning object, becoming increasingly visible before sunrise as opposed to after sunset.

So, enjoy the wonderful sight of Venus in the evening sky over the next 6 weeks or so. And, if you can get hold of a pair of binoculars or a small telescope, you will see that Venus exhibits phases. Currently it is a quarter phase (half of it is illuminated), but as it approaches inferior conjunction it will become more and more crescent, but also appear to get larger in your viewing device (this cannot be seen with the naked eye). It was observations like these which enabled Galileo to show in 1610/1611 that Venus could not be orbiting the Earth, but that both Earth and Venus must be orbiting the Sun.

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My latest book, Astrophotography, is now available. You can order a copy by following this link. Astrophotography is a book of exquisite images of space, including some of the latest images such as New Horizons’ images of Pluto, Rosetta’s images of Comet 67P, and Hubble Space Telescope images of the most distant galaxies ever seen. Each stunning image, reproduced to the highest quality, is accompanied by text that I have written to explain the object, and any background science relating to the object.


Astrophotography is now available. You can order your copy by following this link.

One unique aspect of Astrophotography is that it emphasises the multi-wavelength approach taken to understanding astronomical objects. For millennia we could only study the Universe in visible-light (the light to which are eyes are sensitive), but for the last few decades we have used every part of the electromagnetic spectrum from radio waves to gamma rays to better understand the Universe. This multi-wavelength approach has also enabled us to discover previously unknown aspects of the Universe such as the Cosmic Microwave Background, the true appearance of Venus’ surface which lies hidden below its thick atmosphere, and huge quantities of gas between galaxies (the intracluster medium) which emit no visible-light but prodigious amounts of X-rays.

Astrophotography is split into 5 sections, namely

  1. Exploring the Solar System
  2. Exploring the Milky Way
  3. Exploring the Local Group
  4. Beyond the Local Group
  5. At the Edge of the Universe

Below are examples of some of the beautiful images found in Astrophotography, along with examples of the accompanying text. At the beginning of each page’s text I caption which telescope or space probe has taken the main image, and at which wavelength (or wavelengths).

Exploring the Solar System

Two examples from the first section of Astrophotography, the section on the Solar System, are stunning images of Mercury and of Mars. The images of Mercury were taken by NASA’s MESSENGER spacecraft. There are several pages of images of Mars, the page shown below shows an image of the Martian surface taken by the Mars Curiosity Rover, and an image of Victoria Crater taken by the Mars Reconnaissance Orbiter.


Images of Mercury taken by NASA’s MESSENGER spacecraft. The four main images are spectral scans, and show information on the chemical composition of Mercury’s surface.

The section on the Solar System also includes images of Pluto taken by New Horizons, images of Saturn and Titan taken by the Cassini space probe, images of Comet 67P taken by Rosetta, and images of Jupiter and her moons taken by the Galileo space craft.


The surface of Mars as imaged by NASA’s Mars Curiosity Rover and, at right, Victoria Crater, as imaged by NASA’s Mars Reconnaissance Orbiter.

Exploring the Milky Way

The second section of Astrophotography includes images of the Orion Nebula (Messier 42), the reflection nebula Messier 78, the Horsehead Nebula, the Pillars of Creation (part of the Eagle Nebula), and the Crab Nebula, the remnant of a supernova which exploded in 1054.

The example I show below is of the reflection nebula Messier 78, and is a visible light image taken by the Max Planck Gerzellschaft Telescope, a 2.2 metre telescope located at the European Southern Observatory’s facility in La Silla, Chile. The text describes the history of observing Messier 78, and explains what produces a reflection nebula.


The reflection nebula Messier 78 imaged in visible light by the Max Planck Gesellschaft Telescope. The text explains what reflection nebulae are, and the history of observing this particular object.

Exploring the Local Group

The third section of Astrophotography looks at the Local Group, our part of the Universe. The Local Group includes our Milky Way galaxy, the Large and Small Magellanic Clouds, and the Andromeda galaxy. Some of the images shown in this section include the Tarantula Nebula in the Large Magellanic Cloud, NGC 602 (in the Small Magellanic Cloud), the Andromeda galaxy, Supernova 1987A and the Seahorse Nebula.

The example I show here is the Seahorse Nebula, a dark cloud of gas and dust located in Large Magellanic Cloud. This Hubble Space Telescope image was taken in 2008, and the nebula is in the bottom right of the image.


The Seahorse nebula is a dark cloud of gas and dust found in the Large Magellanic Cloud, an irregular galaxy visible to the naked eye and in orbit about our Milky Way galaxy. The seahorse nebula is in the bottom right of the image.

Beyond the Local Group

The fourth section of Astrophotography looks at the rich variety of galaxies found beyond our own neighbourhood. Examples are galaxies like Messier 82, which is undergoing a huge burst of star formation in its centre, Centaurus A, which shows huge lobes of radio radiation stretching far beyond the stars we see in visible light, colliding galaxies such as The Antennae galaxies, and evidence for dark matter such as the Bullet cluster.

The example I have shown here is the spread for Messier 81, a beautiful spiral galaxy found in Ursa Major. It is one of the best known galaxies in the sky, and is visible to northern hemisphere observers throughout the  year. The main image illustrates the multi-wavelength approach astronomers take to studying many objects. The image combines visible light, infrared light and ultraviolet light to teach us far more about the galaxy than we would learn if we only looked in visible light.


Messier 81 is a beautiful spiral galaxy found in Ursa Major. Hence it is visible throughout the year to northern hemisphere observers. The main image shown here is a combination of of a visible light image (taken by the Hubble Space Telescope), an infrared image taken by the Spitzer Space Telescope, and an ultraviolet image taken by Galaxy Evolution Explorer (GALEX).

At the edge of the Universe

In the final section of Astrophotography, I show examples of some of the most distant objects known. Images include the Hubble Deep Field, the Cosmic Microwave Background, the most distant galaxy seen (GN-z11, lying about 13.4 billion light years away), gravitational lenses and the recent discovery of gravitational waves made by LIGO.

The example I show here is the spread about the gravitational lens SDP81, a galaxy lying about 12 billion light years away which is being lensed (and brightened) by an intervening cluster of galaxies which lie about 4 billion light years away. The top image was taken at millimetre wavelengths by the Atacama Large Millimetre Array (ALMA), the bottom image in visible light by the Hubble Space Telescope.


Gravitational lenses enable us to see distant galaxies which would otherwise be too faint to see, but they also provide us with a way of tracing the distribution of dark matter in clusters.

I hope these few examples from Astrophotography have whetted your appetite to find out more. I really enjoyed putting the book together, and am very pleased with the quality of the images and their aesthetic beauty.

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On Sunday 22 May, Mars will be at opposition. This is the time when Earth passes Mars in their respective orbits around the Sun, and so Mars is at its closest to us. Because it is at its closest to us it is also at its brightest; an opposition of Mars usually presents the best time to see the planet. Earth, of course, takes 365.25 days to orbit the Sun. Mars takes 687 days, this is known as is its sidereal period. The time between each opposition is 779.96 days, which translates to 2 years and 49.5 days, or just under 2 years and 2 months. This is known as the synodic period for Mars.

For this particular opposition, Mars is between the constellations Libra and Scorpio. Below is a diagram showing where Mars will be at 23:00 BST (11pm) on the night of Sunday 22 May as seen from Cardiff. Unfortunately, for places as far north as Cardiff (which is at a latitude of 51.5 degrees north), this part of the sky never gets particularly high above the horizon. In this diagram, at 23:00, Mars only has an altitude of 11 degrees above the horizon, which is quite low. The highest Mars will appear in the sky during this opposition is at around 1am, then it will  be about 17 degrees above the horizon. This is still quite low but, thankfully due to Mars’ brightness, it should be easily visible.


The position of Mars at 23:00 on Sunday 22 May as seen from Cardiff. Mars is in the constellation Scorpio, at about 1 o’clock from Antares if you imagine a clock face. At about 8 o’clock from Mars and about 10 o’clock from Antares is Saturn. On Sunday, the Moon will also be nearby, and as it is near full it may impede viewing Saturn.

Although the opposition happens on Sunday 22 May, do not worry if you are not able to see Mars that night (due to cloud or some other reason). Mars will remain bright in the sky for the next several weeks. In fact, if you have been looking at Mars over the last few months you will have noticed that it has brightened considerably as it has approached opposition. Of all the planets, Mars is the one which shows the biggest change as the Earth approaches it, far more than the others.

For example, in late January Mars was nearly 20 times fainter than  it will be on 22 May. Even in late March it was nearly 6 times fainter, now it has brightened to outshine everything else in that part of the sky. But, it will still be pretty bright for the rest of May and into June. If you are lucky enough to be going away anytime between now and mid-June to more southerly latitudes, then Mars will appear much higher in the sky. People in the Southern Hemisphere are getting a much better view of this opposition than those of is in northern latitudes.

Because Mars will be so bright, it should be very easy to find. It has a distinctive red appearance, but not quite as red as Antares, the red giant star in Scorpio. You can use Mars to find other objects nearby. Imagining a clock face, Antares is at about 8 o’clock from Mars, and about 20 times fainter. At about 9 o’clock from Mars and 10 o’clock from Antares is Saturn, which is currently about 8 times fainter than Mars. On the night of Sunday 22 May, the Moon will be quite close to Saturn, and it will also be nearly full. Although this shouldn’t affect your being able to see either Mars or Saturn, you may find that you get a better view of both if you look a few days before or later, when the Moon won’t be nearby.

By late June Mars will have faded to being about 2 times fainter than it will be at opposition, but still a lot brighter than it was in March or April. By July it will have faded further, being about 3 times fainter than when at opposition, and about the same as it was in late April. But, if you are waiting for your summer holidays to get to southern latitudes, Mars will be easily visible in late July and August, and will still be brighter than either Antares or Saturn.

Do oppositions vary?

But, not all oppositions are equal. The planets do not orbit the Sun in circular orbits, but rather in elliptical orbits. We measure how far from being circular an ellipse is by something called an ellipse’s eccentricity; an eccentricity e=0 means that the ellipse is a perfect circle (a circle is just a special case of an ellipse), an eccentricity greater than this means it deviates from a circle. The eccentricity of the Earth’s orbit is e=0.017, pretty close to being a circle. However, Mars has a more elliptical orbit, the eccentricity of its orbit is e=0.09. The only planet with a more eccentric orbit is Mercury. The eccentricity of Mars’ orbit means that the distance between Earth and Mars when they are at opposition varies. For this particular 2016 opposition the distance will be 76.3 million kilometres (about 47 million miles). The figure below shows the distance between Earth and Mars for all the oppositions between 2001 and this one in 2016.



Oppositions of Mars from 2001 to this current 2016 opposition. The figure shows the dates of these oppositions, and also the distance in millions of miles. For this 2016 opposition, the distance will be about 47 million miles, or about 76 million km

During the August 2003 opposition, Mars got to within 55.8 million kilometres. This was, in fact, the closest opposition for many thousands of years; it has been calculated that the last time Earth got this close to Mars was in the year 57,617 BC! The next opposition which will be as close as the 2003 opposition will be in 2287, quite a wait.

Oppositions from 2012 to 2027

Below is a table of the dates of oppositions of Mars from 2012 to 2027, showing for each opposition the constellation in which Mars appears, its distance from Earth (in both Astronomical Units and millions of kilometres), and its apparent magnitude and apparent angular size (in arc seconds). Remember, apparent magnitude is a negative system, a lower number means brighter. As the figure above shows, the closest opposition that has occurred recently was in August 2003. Close oppositions happen when Mars is near its perihelion (closest to the Sun), because this will also mean that it is closer to Earth. These are called perihelic oppositions, and the time between perihilic oppositions is either 15 or 17 years (it alternates).

As the table below shows, the opposition in July 2018 will actually be better than this 2016 opposition, with Mars getting to within 57.5 million kilometres and brightening to a magnitude of -2.8 (which is twice as bright as during this opposition). However, it will be in the constellation Capricorn, which is even further south in the sky than Scorpio; so even lower in the sky for those of us living in northern latitudes.


Oppositions of Mars from 2012 to 2027

Opposition Date


Distance (AUs)

Distance (million km)

Apparent magnitude

Apparent size 


2012 March 3






2014 April 8






2016 May 22






2018 July 27






2020 October 13






2022 December 8






2025 January 16






2027 February 19






How has Mars brightened over the last year and how will it fade over the coming months?

For those of you who have been looking at Mars over the last few months, you will have noticed how much it has brightened. Here is a table showing the apparent magnitude of Mars on the 21st of each month. If you do not manage to see it during May do not worry, it will remain fairly bright well into June and July and, even in August, it will be outshining Antares and Saturn. If you are heading further south during the summer months, then it would be an ideal time to see Mars.


The distance, apparent magnitude and size of Mars from June 2015 to December 2016







Apparent Size 


2015 June 21





2015 July 21





2015 August 21





2015 September 21





2015 October 21





2015 November 21





2015 December 21





2016 January 21





2016 February 21





2016 March 21





2016 April 21





2016 May 21





2016 June 21





2016 July 21





2016 August 21





2016 September 21





2016 October 21





2016 November 21





2016 December 21






Hopefully you will get a chance to see Mars over the next few weeks or months. If you get a chance to look through a small telescope or binoculars you may be able to make out white near the poles of Mars, these are polar ice caps and are mainly frozen water, with some frozen carbon dioxide. The frozen carbon dioxide has been measured to sublimate into the atmosphere when that particular polar region is in summer, but the water remains permanently frozen. When the pole loses its sunlight, some of the carbon dioxide in the atmosphere freezes again, forming part of the polar cap.



Mars during the January 2010 opposition. Image credit Alan Friedman.

Here is a picture I took of Mars over Cardiff City Hall on Sunday night/Monday morning (15-16 May), at 01:30. For details of the exposure time and aperture etc., see the caption.



Mars over Cardiff City Hall on Sunday night/Monday morning (15-16 May 2016). Mars is just to the right of the clock tower. To the left at about the same height is Saturn, and to the left and low down is Antares. This picture was taken at 01:30 and is a 10s exposure with f/4.4, 44mm focal length and ISO 100 using a Nikon D3200 DSLR.

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With winter (in the Northern Hemisphere) approaching, I thought it was about time I gave a summary of which planets are visible over the next few months. The longer nights, enabling easier viewing of the night-time sky, is one of the few pluses about this time of year as far as I am concerned. So, which planets are visible this winter (2015/16)?

The times I will give for various planets rising or setting are for Cardiff, where I live. So, if you are living elsewhere the times will almost certainly be different. Obviously, if you are living in the southern hemisphere you are about to move into summer not winter. But, although times may vary depending on your location; whether a planet is visible or not, and whether it is visible in the evening after sunset or in the morning before sunrise will not be different.

Of the 5 naked-eye planets, all but Saturn are visible this winter. Here is more detail about each.


Mercury is currently in Sagittarius, rising before the Sun and thus setting after the Sun. So, it is currently an evening object. It reaches maximum elongation on the 29 December when it will be 25^{\circ} to the East of the Sun, and on this day it will set in Cardiff at 17:43. The Sun sets on this day at 16:11 in Cardiff, giving some 1.5 hours after sunset to see Mercury. Although these setting times will vary depending on your location, what will not vary is the time between sunset and Mercury setting, which will be about 1.5 hours no matter where you live.

1.5 hours between sunset and Mercury setting it very good. Mercury is rarely this far from the Sun; so for those of you who have never seen Mercury, this month of December provides a very good chance. Find a view to the western horizon which is uninterrupted and away from city lights, and use the chart below to find Mercury. It will be reasonably bright, at a magnitude of -0.5.

Mercury just after sunset as seen from Cardiff on 29 December 2015. This month is a good month to see Mercury, as its maximum eastern elongation (the maximum angle between it and the Sun) is nearly as large as it can ever be. There are no bright stars near Mercury at the end of December.

Mercury will reach inferior conjunction on 14 January, whereupon it will reappear as a morning object later in January and February.


Venus is currently in Libra. It is a morning object, very bright before sunrise. At a magnitude of -4.1 you cannot fail to see it. It will reach maximum western elongation on 12 January. You can see it in the diagram below of the sky before sunrise, which also shows where to find Mars and Jupiter. Venus will be visible as a morning object throughout this winter and into the spring.


Mars is currently in Virgo. It is rising at the end of December just after 2am, so is a morning object. In fact, it can be seen in the morning sky along with Venus and Jupiter throughout much of the winter, as the diagram below shows. At the end of December it has a magnitude of +1.3, fainter than nearby Spica, which is at +1.05. Mars will reach opposition on 22 May, by which time it will have brightened to -2.1, so some 23 times brighter; making April, May and June by far the best time to see this planet.

The morning sky at the end of December as seen from Cardiff. Venus, Mars and Jupiter are all visible in the morning sky this winter. Jupiter and Venus are easy to find as they are so bright. Mars is a little trickier, but will brighten as it approaches opposition in the spring


Jupiter is in Leo, and is also currently a morning object. At the end of December it rises just before 11pm. It will be at opposition in early March (8 March), and so in late winter and spring it will be an evening object, but for most of this winter it is better seen in the morning before sunrise.

I like it when one can see Jupiter and Venus at the same time, as it allows one to see how much brighter Venus is than Jupiter. Normally Jupiter is the brightest point-like object in the sky, but when Venus is visible it outshines Jupiter by a factor of 6 or so.


Saturn is currently in Ophiuchus, and this winter is not the time to see Saturn. It will reach opposition in early June (3 June), so spring and summer are the best times to see Saturn in 2016 and over the next few years. It will not become a winter object again for another 14 years or so.

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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, 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 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 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 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 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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