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Levenhuk.com » Good to Know » Astronomical observations in the city

Astronomical observations in the city

The high level of light pollution in the urban sky makes it impossible to observe faint objects, but that doesn’t mean you have to pack your astronomy equipment and go to the countryside, where the skies are much clearer and celestial objects appear brighter and more appealing. First, we need to understand what celestial objects we can observe in urban conditions.

What’s the best telescope for urban observing?

The best telescope for observing in urban conditions is the same telescope that is best for observations in the countryside, with its dark and clear sky. All considerations, including objective lens diameter (aperture) size and ease of use, are valid when choosing a telescope for urban viewing as well. However, if the telescope is too big and heavy, you might have some problems when you want to install the instrument, and it will occupy too much space in your apartment. It make sense to choose a computerized mount with automatic guidance (Go To function). Using traditional methods of navigation, you might face some difficulties due to a lack of visible stars, while mounts with automatic guidance cannot be affected by city glare.

How to select a spot for your observations?

Pavements and roofs heat up during the day and radiate this heat at night. As a result, this heated air flows over these surfaces and negatively affects image quality. The best observation spot in the city would be on the ground or grass, which absorb less heat. For the same reason, do not point the telescope at objects located near roofs, buildings, and streetlights. Close bright streetlights also reduce image contrast and irritate your eyes. If you have no other choice, try to cover your head with a non-transparent cloth. You also need to remember that it’s better to observe a celestial object when it reaches its highest position above the horizon. And besides, urban illumination decreases slightly late at night when building lights and illuminated signs go off.

What to observe?

Lets’ focus on possible targets for overnight observation. Speaking of daytime observing sessions, we have to name the Sun, but you have to remember your safety and purchase a special aperture solar filter before you start.

The Moon. The Moon is the biggest and brightest object in the sky (after the Sun). The Moon’s surface with its craters and mountains looks very impressive, even from the city. The boundaries of light and shadow move from night to night, showing us different parts of the surface.

Planets. Four planets in the Solar System are bright enough to please urban astronomers. To get good quality images, the atmosphere should be very calm. But, even if atmospheric conditions are favorable, the observer has to be patient and wait for those infrequent moments of great visibility.

Our celestial neighbor Venus is the third brightest object in the sky after the Sun and the Moon. Because of its location between the Sun and the Earth, we cannot see its whole surface at once. Venus goes through phases like the Moon; its apparent size significantly varies depending on the phase. Venus is shrouded in a thick blanket of clouds that hides most of the details of its surface.

Mars is small and a quite onerous celestial object to observe. But this planet is definitely worth the effort and time as it abounds with intricate surface details. With favorable atmospheric conditions and a powerful telescope you will be able to see one or two of the polar ice caps and some dark spots, including the famous Syrtis Major Planum. Sometimes Martian dust storms can change the appearance of the surface or even completely hide it from view. The best time to observe Mars comes at intervals of about 26 months, when the planet is closest to Earth and the proximity significantly adds to the apparent size.

Jupiter is the biggest planet in our Solar System. This planet, deservedly called “Urban Gem,” impresses astronomers with its high abundance of varied and changing surface details. You will be able to observe Jupiter's differently colored cloud bands and the famous Great Red Spot (a huge vortex about triple the size of Earth’s diameter). One more reason to point your telescope at Jupiter: its four Galilean satellites, named in honor of Galileo, who discovered them in 1610.

Io, Europa, Ganymede, and Callisto can be seen with a small telescope or even binoculars. The Galilean satellites have orbital periods ranging from two days to two weeks; change in their position can be noticed within an hour. Sometimes the satellites line up on one side of the planet; sometimes some of them are hidden behind Jupiter. From time to time you can notice a small dark spot on Jupiter’s belt – that is the shadow of a satellite moving across the planet.

Perhaps the most spectacular view in the Solar System is Saturn and its rings. Even bright city lights cannot overshadow the beauty of this small but very picturesque crowd favorite. When some random passersby look at Saturn through a telescope, they are so amazed by the view that sometimes they even start to wonder what’s the catch; maybe there is some beautiful small image hidden somewhere inside the telescope. No wonder this object causes such reaction, this far-distant planet looks truly impressive! Saturn’s rings can be easily seen in almost any telescope with magnification of 40-fold or more. A middle- or high-powered telescope will help you to discern faint differences in shades of different parts of the planet, and maybe you will be able to notice a thin black shadow line of rings on the planet surface. When the atmosphere is stable, you can distinguish two rings - the outer gray ring "A" and the inner white ring "B", separated by a thin dark gap, known as the "Cassini Division." Careful observation may reveal a dark gray ring "C" in the ring "B." Observing Saturn regularly, you can notice that ring inclination angles vary within 26 degrees in a period of about 15 years. At times, when the tilt is close to zero, the ring disappears from our view. Saturn has multiple satellites, although most of them are too small, but you can always see the largest of Saturn’s moons - Titan.

Mercury, Uranus and Neptune are also easily reachable for amateur telescopes in urban conditions, if you know where to look. But they are too small, and appear just like stars, and you won’t be able to see any details. Forget about observations of Pluto within the city limits; it’s now excluded from the list of planets, anyhow, and is considered a “dwarf planet.”

Double and Variable Stars. Double and variable stars are fascinating objects to observe. They are bright enough to make their way through high amounts of light pollution around big cities. Each binary star is unique and sometimes forms beautiful multi-colored pairs. Distance between components can be very different. One of the most beautiful binary stars is Albireo in the constellation Cygnus. One component - golden 3rd-magnitude star; the second - 5th-magnitude sapphire-blue star. This object looks fascinating even with a small-aperture telescope. Another interesting example - Epsilon Lyrae (also known as the Double Double) - multiple star system in the constellation of Lyra. At low magnification you can see two widely spaced star components of the same brightness, but, increase your telescope’s magnification power to 100x and each object will break into two separate stars. Variable star observing requires more patience from the observer. Variable stars vary in brightness over time periods, ranging from a few hours to several months or even more. You can detect changes in brightness by comparing your target variable star with conventional neighboring stars. Changes in brightness of some binary stars can be seen with the naked eye, even though they are very slight. For example, the star Algol in the constellation Perseus changes its magnitude from 2.1 to 3.4 every 2.87 days.

Deep-sky objects. Most deep-sky objects are difficult to observe even in dark sky, not speaking of highly polluted urban sky. Galaxies and nebulae are affected the most, a slightly better situation with scattered stars and globular star clusters. However, you will be able to observe some deep-sky objects as well. Let us remind you that the higher objects are located above the horizon, and the later at night you observe, the better! Some examples of bright star clusters: double cluster in Perseus (h and Chi Persei), Hercules Globular Cluster (M13), Wild Duck Cluster in the constellation Shield, Pleiades in the constellation Taurus, M44 in Cancer, M52 in the Cassiopeia constellation; M4, M6in Scorpio and M22 in Sagittarius. Planetary nebulae are small but have relatively high surface brightness. Suitable objects for urban astronomy are The Ring Nebula (M57 in Lira) and The Dumbbell Nebula (M27 in Vulpecula).

The list of nebulae and galaxies that can be observed from the city is very limited. Often, in order to detect a deep-sky object, you have to set the lowest magnification power and use your peripheral vision or even gently tap on the telescope tube to create vibrations, as our vision responds well to any movement. Some relative success can be achieved observing the Andromeda Galaxy (M31), Whirlpool Galaxy (M51), and M81, Spiral Galaxy in Ursa Major. The most suitable objects for urban observations are The Orion Nebula (M42), Lagoon M8 and Swan M17 in Sagittarius. We also recommend using a narrowband filter that suppresses flare to increase the contrast of the nebula on the sky background.

As you can see, light pollution of the cities is definitely not a drawback to your successful observations.