Observing the Planets
Follow the tips below to improve your views of the planets!
1. Planet and Solar System Basics
The Sun, Planets, and Minor Bodies in the Solar System to scale
Our solar system consists of the planets and many smaller bodies orbiting around the sun at the center. Over 99% of the mass of the solar system is contained within the sun.
Planets are bodies of rock and or gas that orbit stars. With the thousands of objects that orbit stars, such as our Sun, planets are reserved for the largest and most prominent orbiting bodies.
The planets in our solar system are split into two main categories terrestrial (rocky planets), and gas giants. The terrestrial planets are small in comparison to the gas giants with the smallest planet Mercury being smaller than one of the moons of Jupiter. Our solar system is rather neatly organized, with the first four planets, Mercury, Venus, Earth, and Mars all terrestrial, and the outer four, Jupiter, Saturn, Uranus, and Neptune being gas giants. Most planetary systems we know of are not well organized, with planets larger than Jupiter closer to their star than Mercury.
2. Terrestrial Planets
Earth is the largest of the inner planets, with Venus a close second at 80% of the Earth's masses, followed by Mars with a little more than 10% of the mass of Earth, and Mercury the smallest at 5 %. If we could observe the Earth from the outside it would be the prettiest and most dynamic of the inner planets.
Venus and Mercury orbit closer to the Sun than the Earth does and so never appear far in the sky from the Sun. Viewing the planets is easiest at night when the glow of the Sun and are most easily seen at their greatest elongation, their farthest apparent distance from the Sun. This means Mercury is only easily seen a few days every 2 months, and Venus is not easily seen for 2 months when closest to the Sun.
Since Mars and the outer planets have orbits farther from the Sun than the Earth, their greatest elongation is referred to as opposition with the planet lying directly opposite in the sky of the Sun.
All of the inner planets have orbital distances with a similar magnitude of the Earth’s orbit, resulting in their distances being able to vary by a factor of 2 or more. As a result, they change in size and are largest when closest and smallest when farthest away. The outer planets also experience a change in size but it is much less drastic and apparent.
Terrestrial Planets to Scale
3. Gas Giants
The outer planets are gas giants, large swirling spheres of gas and ice on the outer lanes of the solar system. All these gaseous worlds have non-zero excess heat meaning that they radiate away more energy than they receive from the Sun. Their storms are driven by the heat that comes from within, with Jupiter having the most excess heat and Uranus the least.
Jupiter is the most dynamic of the gas giants with an ever-changing atmosphere filled with storms and two prominent reddish-brown dust lanes. Saturn has gorgeous rings and occasional large high-contrast storms that rip across its disk.
Uranus observed in visible light is often a pale green featureless disk hanging in space and can appear quite odd in an eyepiece. Neptune is far away enough that its details can not be visually observed and require lucky imaging techniques from the ground.
All the gas giants have many moons, some of which can be seen when viewed with a telescope. Jupiter has four large moons that are named after Galileo for discovering them. Saturn has a handful of larger moons such as Titan, Iapetus, and Rhea. Uranus and Neptune may have one or two moons that are visible to the eye using large telescopes.
The Gas Giants to Scale
4. Viewing the Planets
The word planet comes from an ancient Greek word meaning to wander and they can be seen moving against the background sky day to day as they move in their orbits around the Sun. The naked eye planets appear as points of light to the unaided eye but do not twinkle like stars do. If you see a star that appears not to twinkle while the stars around it are, you are seeing a planet. I personally place the planets into three tiers from best to worst in regards to visually observing them.
Best: Venus, Jupiter, Saturn
Venus, Jupiter, and Saturn are always stunning. Venus dazzles brilliantly bright, Jupiter has vibrant colorful cloud belts, and Saturn has gorgeous rings. These planets are always a joy to observe and will always remember your first view of them in a telescope.
Interesting but Difficult: Mars, Mercury
Mars and Mercury I will plan to look at. Mars needs great seeing, well-collimated optics, and appropriate filtering to really see surface detail. These planets often require dedicated planning or longer setup times, and Mars only looks great near opposition. Mercury is always low in the sky and requires planning to set-up at a location with visibility of or near the horizon.
Dim and Difficult: Uranus, Neptune
Uranus and Neptune do not need any additional setup but well-collimated optics are a definite plus. Since they are not visible to the naked eye they take a while to find and with no surface detail, they are rather uninteresting. They are a fun challenge to find and observe!
Jupiter and the Galilean Moons at 180 x
5. Improving Planetary Views
Planets are bright but they are small in the sky. Planets are typically seen at high magnification and the equipment and environment need to support that.
Larger telescopes can show more detail:
Details in objects in space are limited by the resolution of the telescope and the seeing conditions. The diameter of the aperture of a telescope is linearly proportional to its resolution, with a larger telescope providing a higher resolution. An 8-inch telescope has a resolution of about 1 arcsecond while a 16-inch telescope has a resolution of about 0.5 arcseconds. One arcminute is 1/60th of a degree and one arcsecond is 1/60th of an arcminute. This means that a larger telescope will be able to resolve and show more detail if the optics are properly aligned. A large telescope with unaligned optics will not have great views.
Observe Under Excellent Seeing:
The best nights to view the planets are few and far between due to thee rarity of excellent seeing conditions. The seeing dictates the ability to see detail and is split into two components, transparency and stability. Poor transparency is caused by smoke or high-altitude clouds and reduces the contrast of a planet’s surface. The stability in the sky is dictated by the size of the turbulent eddies in the upper atmosphere and typically ranges from 0.5 to 2 arcseconds. When the air is stable the stars will appear still and will not be twinkling. When the seeing is poor the brightest stars will appear as wobbling rainbows. The most transparent and stable air will be directly overhead and so planets are best viewed when they are high in the sky. Unfortunately, still nights have low transparency and vise versa. Early mornings before sunrise have in my experience been some of the best seeing conditions I have observed in as evenings tend to be windy until midnight.
If the telescope is not acclimated and at the same temperature as the environment turbulence inside the telescope will make the seeing appear poor. Allow adequate time for the telescope to equalize with ambient air conditions.
The planets range is apparent size from 60 arcseconds to 2 arcseconds, with major features on the planets ranging between 5 to 0.5 arcseconds. This makes planetary viewing for telescopes over 8 inches seeing limited with the more minute details only able to be seen during brief moments of excellent seeing.
View Near Opposition:
The planets with the exception of Mercury and Venus are their biggest and brightest when they are opposite the Sun. During opposition, a planet is directly opposite the Sun in the sky, rising at sunrise and setting at sunrise. Around opposition is the best time the best time to view the planets.
The gas giants do not vary much a month before and after opposition except for Saturn with a brightening of its rings for a day before and after opposition called the Seeliger effect. Mars, however, changes in size and brightness dramatically, more than doubling in size and appearing over 50 times brighter between solar conjunction and opposition.
6. The Phases of Venus
Venus is the third brightest object in the sky only beaten by the Sun and Moon. Since it is always close to the Sun in the sky and how bright the planet is, it can not be seen in the middle of the night and is often the first object visible at dusk. As a result, it has earned the name the evening or morning star.
All planets can be seen going through phase changes whenever they are not in opposition. Venus orbits the Sun closer than the Earth allowing for all of its phases to be observed. Seeing Venus as a full disc or thin crescent is difficult and risky as it is close to the Sun. It is possible but should only be done with the proper equipment and planning.
The phases of Venus are easily seen from a gibbous phase to a crescent when the planet has a greater angular distance from the Sun. Venus is always brilliantly bright naked eye or with a telescope when not viewed during the day.
Venus can actually be seen with binoculars or a telescope during the day. It can be a challenge to find without a go-to system but it is doable. Seeing detail in the clouds of Venus is difficult, and like all planets to maximize detail, they should be observed when highest in the sky. Venus is always highest in the sky during the day and with the aid of a yellow planetary filter such as the W15, some details can be seen.
Venus at Different Phases in a Telescope
7. Juptiter’s Moons and Dust Lanes
Jupiter is the most dynamic planet to observe in a telescope. Over the course of a few minutes, you can see the Galilean moons progress in their orbits, and with Jupiter’s fast rotation period of about 10 hours, you can see the entire surface in one night.
Jupiter has two characteristic reddish-brown dust lanes that cross its atmosphere and they can be seen even at low magnifications. The famous great red spot (often referred to as GRS) requires high magnification to see and the use of a blue planetary filter such as the 82 A can help it stand out.
With medium-power telescopes the Galilean Moons, Ganymede, Callisto, Io, and Europa are all indistinguishable but with a large telescope at high magnification, their differences in color and size can be seen.
Earth is not the only planet to have eclipses, with Jupiter’s moons casting shadows on the Jovian atmosphere as they pass in front. These are called shadow transits, and multiple can occur at a single time. Io, with its shortest orbit, has shadow transits the most often. Ganymede, with a longer orbit, transits less often but casts a larger shadow that is easier to see. Jupiter just as often eclipses its moons, and they can be seen popping in and out as they pass behind the gas giant. Sky and Telescope has a great tool for Jupiter’s moons’ current locations. Other planetarium apps and software such as Sky Guide have accurate simulations as well.
A Shadow Transit on Jupiter Through a Telescope
8. The Rings of Saturn
Saturn is a planet that is always gorgeous to look at. It is rightfully the favorite planet of many with its dazzling rings. Saturn and its rings are about the same as Jupiter’s entire disc, and the urge to make it appear larger in the eyepiece is always there.
Saturn is possibly the next object to show that more magnification does not mean a better view. Saturn when smaller will appear sharper and there is a sweet spot for seeing the most detail, not too small and not too large.
The moons of Saturn can also cast shadows on the surface, but shadow transits are much rarer and smaller than on Jupiter.
The rings of Saturn are not a single object but actually a countless number of particles from the size of dust grains to houses orbiting the planet. The rings are also split into sections and have gaps between denser regions. The largest and most prominent gap in the rings is the Cassini division caused and maintained by the gravitational effects of the moon Mimas. A 6-inch telescope or larger is recommended to see the Cassini division. A smaller gap in the outer edge of the rings called the Encke division requires at least a 6-inch telescope and excellent seeing to see. Larger telescopes will make seeing these divisions easier to see.
Saturn and its Moon Through a Telescope
Change in the Apparent Angle of Saturn’s Rings Opposition to Opposition
The apparent angle of Saturn’s rings, the angle that they appear to us, changes as the planet moves around the Sun. The angle of the rings are an indication of the tilt of the planet relative to the plane of the solar system with this tilt causing seasons just like on Earth. The rings are at their largest tilt during a Saturnian solstice and obscure one of the poles. During the Saturnian equinox the rings appear edge on to us on Earth and can appear to vanish for a few days. The next time this will occur is May 6th, 2025.
9. Canyons, Clouds, and Ice Caps on Mars
Mars is the only terrestrial planet that from Earth shows surface detail when viewed visually through a telescope. Venus shows minimal detail of its cloud tops, and Mercury is too small and close to the Sun to resolve surface details.
Mars despite being closer to Earth than Jupiter, never has an apparent size larger than it due to its small diameter. During opposition Mars is about as large as Saturn’s disc alone and when near solar conjunction is almost as small as Uranus.
To see surface detail on Mars it needs to be observed near opposition with a large telescope under excellent seeing conditions. During opposition, even when using small exit pupils and high magnification, the planet is so bright that it blows out the details on the surface. A polarizing moon filter can help reduce the surface brightness to a level that allows observation of surface details. A light blue filter such as an 82 A can help improve contrast.
Darker and lighter regions are easily seen on the surface, and under good seeing conditions, the polar regions can shine white. Mars has seasons just like Earth, and as Mars progresses in its orbit, the polar ice caps can be seen growing and shrinking over the course of a few months. Depending on the Martian weather large clouds are dust storms can be seen as well. Under excellent seeing conditions and with a large telescope Valles Marineris, the largest canyon in the solar system can be seen.
Mars during Opposition in a Telescope
10. Finding Mercury, Uranus, and Neptune
Mercury, Uranus, and Neptune are all much harder to observe than the other planets.
Mercury is small, always close to the Sun, and undergoes phases that are visible during its entire orbit just like Venus. To see Mercury view it during its greatest elongation when it is its farthest apparent distance to the Sun. Set up a telescope at an area that has visibility of the horizon in the direction of Mercury and start looking as soon as it rises if viewing in the morning or within 10 minutes of sunset. Mercury is not dim but against the background glow of a twilight sky, it can be hard to find. For the best views, understand the seeing conditions in your area and choose a location and time with the best seeing possible. Mornings tend to have less wind and still air than evenings, producing better seeing conditions. Avoid looking over buildings as the heat from a building will create turbulence in the air and worsen the view.
Mercury in a Telescope in the Twilight Sky
Use a star chart of planetarium software to find Uranus and star hop to its location. Under magnification greater than 100 x, it should appear different than the background stars and be a resolved pale blue-green disk. It will appear as a featureless sphere sitting in space. Uranus is resolvable as a disk in 4-inch and larger telescopes. Good seeing conditions are a help but are not necessary to see the planet as different than a star. Uranus is visible to the naked eye only under dark skies.
Uranus in a Telescope Under Good Seeing
Just like Uranus, a star chart is needed to find Neptune. Neptune needs a telescope of at least 8 inches of aperture to be distinguishable from nearby stars. At a little less than 2 arcseconds across, Neptune will appear no different from the surrounding stars in small telescopes or in poor seeing conditions. At 180 x and above Neptune should appear as a resolved disk, larger and different in shape than the background stars. Stars at magnifications near the diffraction limit (resolution limit) of a collimated telescope\ should appear to have faint concentric circles around them. Neptune will not appear to have this diffraction pattern under good seeing conditions. The planet will also lack a luminous white center like similarly bright and colored stars.
Neptune in a Telescope Under Good Seeing