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Jan Ströher

Jan Ströher

Posts composed by Jan Ströher

The gas giants at opposition

July 13 2020, Jan Ströher

Clear summer nights in July should be used for observing the large gas planets Jupiter and Saturn, as both are at opposition to the Sun in the middle of the month. For us in central Europe this creates optimal conditions as both planets will reach their greatest apparent magnitudes and their highest positions above the horizon.

Unfortunately, the planets are usually found very close to the horizon, impairing observation due to strong air turbulence (so-called “seeing”). Furthermore, a position close to the horizon also results in a restricted field of vision due to buildings, mountains or trees. The atmospheric turbulence can be counteracted using an ADC (“Atmospheric Dispersion Corrector”).

At the time of opposition, both planets are located towards the south, in the constellation of Sagittarius, around 21° above the horizon which makes observation considerably easier. Due to their maximum apparent magnitude at this time, the gas giants are very easy to locate.  A star chart can offer good guidance.


The gas giants are also great viewing targets for newcomers or children with their parents’ support!

Jupiter will shine at up to magnitude -2.7, Saturn at around magnitude +0.1. Additionally, the giants will converge in the sky at a distance of around 7°. Both will, therefore, be able to be observed together over an extendedperiod of time, even during the short summer nights. In the case of Jupiter, smaller telescopes can also detect its four Galilean moons, Europa, Ganymede, Io and Callisto, as well as enabling examination of the planet’s main cloud bands. To increase contrast on the cloud bands, a suitable filter, such as a polarising or colour filter is often used.

Jupiter in March 2015 (Photo: Berns Gährken, Munich in March 2015 using a C11)

The much more distant Saturn will show us its impressive ring system and, in larger optics, also its Cassini division as well as the largest four or five of its total 82 moons.

Saturn surrounded by the moons Enceladus, Tethys, Dione, Rhea and Titan (Photo: James Bates, Berlin August 2019, C8, 2x Barlow, ADC, UV/IR band-elimination filter, ZWO ASI 224MC)

The precise opposition dates are 14 July for Jupiter and 20 July for Saturn. The months of July and August are generally ideal for observations – also in the periods directly before and after the opposition dates.

Those who subsequently have not had enough of planet watching can look forward to the commencing Mars opposition from September. The red planet is much closer to us, but also much smaller than the giants Jupiter and Saturn…

You can also show your enthusiasm for the planets with a suitable summer outfit.

We wish you a clear sky, success and fun when observing!

Good conditions for observing Venus

January 20 2020, Jan Ströher

In the coming weeks Venus, our “sister planet”, will become a good object for observing. The planet is a bright, easily detectable object in the morning or evening sky, but it is usually located very close to the horizon with corresponding atmospheric disturbance and rather short observing times. This will improve from around the end of January, when Venus will become progressively brighter and visible for longer in the evening sky. Then the planet will be found easily with the naked eye immediately after sunset and can be observed for almost four hours.

Even good binoculars, such as the Omegon Nightstar are suitable for observing. In telescopes with an aperture from around 90mm, Venus can already be identified as a small disc. Just like the Moon, the planet exhibits different phases, although details of the surface remain hidden owing to its very dense atmosphere. The cloud structures can be distinguished very well with telescopes from 130mm aperture. The use of a suitable filter (violet, dark blue, blue) is recommended to improve the contrast.

Credit: EXAME/JAXA/Divulgao, Brazil

Venus is the second innermost planet in our solar system and is a similar size to Earth. Its atmosphere consists of carbon dioxide, nitrogen, sulphur dioxide and various noble gases. This composition, combined with its proximity to the Sun, makes our neighbouring planet a hostile and mysterious world. As it orbits the Sun, Venus rotates backwards, that is in exactly the opposite direction to our Earth. Therefore on Venus the Sun rises in the west and sets in the east. After the Moon, the planet is the brightest object, but, just like Mercury it is only visible in the morning or evening sky – hence it is also given the designation “morning-” or “evening star”.

From mid-January, Venus dominates our evening sky immediately after sunset on the southwestern horizon. Between then and the end of March it changes its position from about 25° to 46° and reaches a brightness of -4.7mag by the end of April. During this period it moves towards the western horizon and passes through the constellations Aquarius, Pisces and Aries. In April it reaches Taurus and can even be found close to the Pleiades (M45) at the beginning of April.

From January to May it’s best to track Venus using a star chart.

Credit: Planetarium Bochum

Have fun observing Venus in 2020!

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