Monthly Archives: September 2016

How to Observe Jupiter for Beginners

Published / by Nick

There are very good reasons why Jupiter is known as the “amateur astronomer’s planet”, chief among which are the facts that no two views of the planet are ever the same, and that the rich detail visible in its cloud tops can literally change before the observer’s eyes. Add to this the facts that Jupiter is visible for about 360 days out of every apparition that lasts 398 days, that it is very bright, and that its apparent angular diameter varies between 30, and 60 seconds of arc, and it should be obvious that the King of the planets is a spectacular sight even when using modest beginners skywatching equipment. If you want to see Jupiter like it appears in the image below, here is how to do it.

Image credit: Sean Walker

Image credit: Sean Walker

Finding Jupiter

Although Jupiter is a spectacular sight, bear in mind that it is very difficult to observe from the UK during September 2016. For instance, at the time of writing, September 3rd, Jupiter sets at 20:16 Local time as seen from London. By month’s end, Jupiter will set even earlier, at 18:39 Local Time, which will make it almost impossible to observe low on the western horizon. The planet will only become easier to spot around the middle of October 2016, at which time it will become an early morning (pre-dawn) object.

Nonetheless, despite the current difficulties in spotting Jupiter, it will dominate the pre-dawn sky for many months after it reappears, but getting up before dawn to view the planet is well worth the trouble, although there are some things to keep in mind to get the best views, such as:-

Bigger telescopes are better

Although Jupiter will consistently show some detail in small telescopes and even binoculars, it must be remembered that even though the planet is big and bright, it does not tolerate large magnifications very well.

For this reason, it is recommended that a telescope with a minimum of 8 inches of aperture be used to view Jupiter. Since a relatively large instrument gathers more light than smaller apertures, lower magnifications can be used to highlight details such as the equatorial and other belts, and even the Great Red Spot if seeing conditions allow.

As a general rule, magnifications of between 50× and 100× give the best results on 8-inch and larger instruments, but when it comes to observing the planets, atmospheric conditions are often more important than the aperture of a telescope. In some cases, and depending on the actual instrument used and local seeing conditions, you may require a slightly larger magnification, but again as a general rule, magnifications in excess of 200× or so tend to “soften” views of Jupiter, causing a marked loss of detail or sharpness in the view.

Filters are required for the best views

Note that the image below is for reference purposes only. It is highly unlikely that a view such as this will be seen through a small to medium telescope, but views approaching the one shown here might be possible with 14-inch and larger instruments, and then only in excellent seeing conditions. Also, note that by not using suitable filters, much of the colour variations and contrasts shown here will be lost to the observer.

Image credit: www.astronomy.com

Image credit: Astronomy.com

Typically, the best results are obtained with filters that are of an opposite colour to the feature an observer wants to enhance. For instance, reddish brown belts and the Great Red Spot are best seen with a blue (Wratten #82A, #80A, and #38A) filters, while red (Wratten #21, #23, and #25) filters work best to accentuate blue features. Similarly, yellow (Wratten #12, and #8) filters work well to accentuate the Polar Regions, where there is less contrast than elsewhere on the planet. Note that this image of Jupiter is upside down, with South at the top, which is how many, if not most telescopes render images.

Some Galilean moons are always visible

Named after their discoverer, Galileo, who first observed them in the early 1600’s, Jupiter’s major moons Io, Ganymede, Europa and Callisto, are almost always visible, and the best part is that they are visible even in modest binoculars.

Image credit: en.wikipedia.org

Image credit: Wikipedia.org

The image above shows the four Galilean moons through a 10-inch reflecting telescope, but from this image, it is not immediately clear which moon is which. There are many online resources available that tell an observer which moon is where, and at what time, but the major attraction of observing these pinpoints of light is to watch them as they weave their way around their parent planet.

Indeed, it is possible to watch their arrangement around Jupiter change from one hour to the next, and long observation shows them transiting the planet, or being occulted by the planet’s disc. A transit, which is when a moon moves across the planet’s disc, is the most fascinating of these events, because when the transit is tracked, the observer is reminded of the fact that nothing in the solar system, or indeed the Universe, is stationary. Everything revolves around something else, and by observing Jupiter and four of its many moons, this principle is demonstrated in the most vivid way possible.