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Most of the planets in our solar system can be observed over the course of a year and this guide will try to inform you of the best way to observe them. Some are difficult objects which require some care when observing them and others can be easily seen with the naked eye, so lets start with the closest planet to the Sun Mercury and work our way outward.

Because the Planets orbit the Sun their position against the background stars will change over time, and with the inner planets Mercury and Venus, these changes can be seen over the course of a few days.

There are many ways to find the position of the planets in the sky and some of these include, astronomy magazines which give a guide to the various planets and objects that can be seen during a particular month, internet sites of which there are hundreds and free mobile phone and computer applications such as Stellarium.

For the purpose of this guide we will use a standard distance of 1 Astronomical Unit or 1 AU. This is the average distance from the Sun to our planet Earth. We use the average distance because all of the planets orbit the Sun in elliptical orbits and are therefore sometimes further away than at other times. For Earth this measurement is 150,000,000km or 93,000,000 miles and therefore 1AU is a much more convenient measurement.

As with all astronomical observations, what you see will depend on several factors most of which are determined by the British weather. Some are dependent on the telescope that you use but as a guide start with a low magnification, and go up to a maximum magnification equal to the diameter of your telescopes aperture in millimetres. This would therefore equate to 80x for an 80mm scope, 150x for a 150mm scope and so on. You can on very clear stable nights reach 2x this amount so 300x for the 150mm telescope. Usually however it will be somewhere between these two figures. To calculate the magnification simply divide the focal length of your telescope by the focal length of the eyepiece. Therefore a 10mm eyepiece in a 1000mm telescope will give 100x magnification.

Observing planets through a thin high mist usually gives excellent seeing as for mist to form the atmosphere needs to be fairly stable.

How much detail you will be able to see depends on the diameter of the telescope. Not the power!

The images below are a simulated view of Mars through three different telescopes and only the diameter of the telescope has changed, this is why professional observatories have large telescopes, (a) to collect the light from dimmer stars and galaxies and (b) to get a better resolution.

Please note that the actual size of Mars appears much smaller than it does in these images.


Mercury 0.39AU

Mercury orbits the Sun at an average distance of 0.39AU and is therefore never very far away from the solar disk. In order to see Mercury you must ALWAYS observe after sunset or before sunrise. NEVER look for Mercury while the Sun is visible and take special care when observing in the morning before dawn as the Sun may rise within the same field of view of your binoculars or telescope. Even the shortest glimpse of the Sun through an unprotected telescope or binoculars can cause permanent damage to your eyes!

Using a current sky chart for your location, look low down near the horizon in the position indicated by your chart a half hour after sunset or before sunrise, and providing the horizon is clear of low cloud or mist Mercury should be seen as a dim star through binoculars.

As Mercury orbits the Sun the amount of sunlight reflected back to Earth will alter in the same way that the phases of the Moon vary in brightness during its orbit around our planet. This will cause the planet to appear brighter or dimmer at different times during its orbit.

Mercury is always a challenge and is often the last planet to be seen by astronomers during their lifetime.
Apparent size 5-13 arc seconds

Venus 0.72AU

Otherwise known as the Morning or Evening Star, Venus is extremely bright and similar to Mercury, it will show phases like the Moon which can be easily observed using a telescope.

With the correct filter fitted to the eyepiece, some detail may be observed within the atmosphere of Venus but this appears most often when combining a telescope with a camera.

To find Venus use a current sky chart as mentioned above.

Because it is further from the Sun it is much easier to see and can be seen long before sunrise or after sunset as the plane of its orbit is at a different angle to ours.

Apparent size 10-64 arc seconds

Mars 1.52AU

Mars orbits the Sun further out than the Earth and because of this it has a longer year than Earth's. This causes the time between oppositions to be around 780 days or 2 years 50 days, as the Earth travels past Mars and then has to complete more than one orbit of the Sun to catch up with Mars again. An opposition occurs when the Sun, Earth and an outer planet are all in line.

Mars appears to move from West to East relative to the background stars, most of the time. However, when the Earth begins to catch up with Mars and overtake it, Mars will appear to stop for a few nights and then start to move in the opposite direction in relation to the background stars. This is known as retrograde motion which can last for a month or so before it comes to a halt again and then reverse direction once more to continue on its normal West to East travel. This motion is common to all of the outer planets and the same effect can be seen from a moving vehicle when a foreground object moves against the background.

Earth's orbit is also more circular than the orbit of Mars, so the distance between the two planets varies from opposition to opposition. At its closest it is around 56,000,000 km (35,000,000 miles) and at its farthest 101,000,000 km (63,000,000 miles). This equates to a disk of 21.8 arc seconds in diameter at its largest (closest) and 13.8 arc seconds in diameter at its smallest (furthest) both seen at opposition. The size of Mars at the 2016 opposition will be 18.4 arc seconds and 24.3 arc seconds at the 2018 opposition although it will appear low down amongst the stars of Capricorn at this time.

So what can we expect to see? Well lots actually. The overall colour is red but some lighter more orange-yellow patches can be seen. Then there are the dark rocky areas which really stand out against the red background. These include Cydonia Mensae and Syrtis Major among many others. White ice caps can be seen at the poles and these can shrink or grow as the seasons on Mars change from winter to summer and back. Often seen at the outer equatorial regions are faint white clouds which relate to the morning and evening mist here on Earth, and to the South there is a huge impact basin Hellas Planitia which can be shrouded in mist and fog.

Occasionally you may be unable to see any surface details at all but don't worry as this is not a fault of your eyes or your telescope but probably due to a global dust storm on the planet.

As Mars is at its best every 2 years, spend lots at time observing its features over the few months that it can be seen.
Apparent size 4-24 arc seconds.

Jupiter 5.2AU

Jupiter really is the king of planets. It is always a rewarding sight in any size of telescope and will even appear as a disk in binoculars. The first thing that you will probably notice are the line of small bright stars that appear on one or both sides of the planet.

These are Jupiter's four largest moons, Io, Europa, Ganymede and Callisto. All four may not be visible when you observe as one or more may be behind or crossing in front of Jupiter.

The other distinctive feature of Jupiter are the cloud belts which cross the planet horizontally. These ever changing belts contain a wealth of detail which can be seen in larger instruments.

The other prominent feature is the Great Red Spot. This is a rotating hurricane like storm which is known to have been active for at least 300 years and can be seen by most people as it crosses the planet's disk as Jupiter rotates in less than ten hours. Also visible are light and dark markings, and sometimes the dark circular shadow of one of the Moons if not the moon itself.

Apparent size 31-48 arc seconds.

Saturn 5.4AU

No matter which telescope you look through you will never forget your first view of Saturn.
Even small scopes will show the magnificent rings for which Saturn is famous. Telescopes of 6 inches will show gaps in the rings such as the Cassini division, and subtle shading on the planets disk. Some of Saturn's moons may be visible but may require steady seeing and larger telescopes.
Apparent size 15-21 arc seconds.

Uranus 19.2AU

Uranus and Neptune can be difficult objects to find unless you have a computerised goto telescope.
Uranus appears as a small pale blue-green 3-4 arc second disk in all but large telescopes, and only looks like a slightly larger blue-green disk in those. Different people see Uranus as a different colour to this but most agree that it is blue.
Apparent size 3-4 arc seconds.

Neptune 30AU

Like Uranus, Neptune appears as a small Blue disk but considerably dimmer but bluer than Uranus. When looking at this small 2.5 arc second blue disk, remember you are looking at our solar system's most distant true planet.
Apparent size 2.5 arc seconds.

Article by Arthur Fentaman

Mid-Kent Astronomical Society
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