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How to Best Observe the Planets with a Telescope

by Zane Landers

Observing the planets is probably the first thing you’re going to want to do with your telescope, as the planets are easy to find, bright, and visible regardless of light pollution conditions. Each planet has a different window for visibility and requires different equipment capabilities to see well, as well as showing different amounts of detail or orbiting natural satellites.

Mercury

Best time for viewing:

Week around greatest elongation

Best magnification:

80x or more

Best aperture:

6” or more

Upcoming greatest elongation dates (Morning):

Upcoming greatest elongation dates (Evening):

Mercury is not known to be the easiest planet to observe, but it’s visible to the naked eye as a bright “star” during twilight when it’s around greatest elongation, or its greatest apparent distance from the Sun in the sky. Mercury never gets more than 28 degrees away from the Sun in the sky, however, so it is best observed during evening or morning twilight within a week on either side of the dates listed above. Mercury will show its phases in a telescope at high magnification, around 80x or more. Mercury’s small apparent size and lack of large high-contrast features means there’s little else to be seen.

Venus

Best time for viewing:

Within around 2 months of greatest elongation dates

Best magnification:

20x or more

Best aperture:

Any

Upcoming greatest elongation dates (Morning):

Upcoming greatest elongation dates (Evening):

Venus’ phases were discovered by Galileo with a telescope about as powerful as your average pair of modern binoculars, and its dazzling brightness makes it easy to spot with the naked eye after sunset or before sunrise (and even in broad daylight if you know where to look). Venus reaches its greatest elongation on either side of the Sun once every 584 days (during what is termed a synodic period), and thus it is usually well-placed at dawn or dusk at least once per year. Venus doesn’t move nearly as quickly around the Sun as Mercury and the Earth is nearly keeping pace with it, so Venus is visible for several months on either side of greatest elongation, though it is at its highest in the weeks around greatest elongation and thus ideally placed for Venus. Due to Venus’ thick cloud decks, which obscure the scorching surface beneath, little other detail is visible besides the phases; large telescopes with colored or polarized filters can theoretically bring out striations in the cloud bands but in practice, only astrophotography will clearly show them.

Mars

Best time for viewing:

Within around 3 months of opposition

Best magnification:

100x or more

Best aperture:

6” or more

Upcoming opposition dates

Mars’ greater distance from us than Venus combined with its small size and elliptical orbit around the Sun mean it’s often quite small in apparent size and thus hard to resolve anything on. However, when around opposition – its closest to the Earth and thus biggest and brightest – Mars swells to an angular size rivaling Venus or Jupiter and appears as a blood red “star” in the sky brighter than any actual stars and only beaten by the Sun, Moon, Jupiter, Venus, and orbiting space stations as the brightest object in the night sky.

The best oppositions occur when Mars is also at its closest to the Sun, which it varies in distance from by nearly 10% thanks to its elliptical orbit (and by extension, varies in its proximity to us at opposition by quite a bit). Unfortunately, Mars’ next few oppositions will be among the poorest in this regard, with Mars reaching only 17 arc seconds in apparent diameter in 2022, 14.5 in 2025 and just 13.8 in 2027, after which it will begin to get bigger and brighter with each succeeding opposition for a while. The best oppositions occur in August or September, the point in the Earth’s orbit that lines up with the perihelion (closest point to the Sun) in Mars’ elliptical orbit.

Through even a small telescope at a less-than-ideal distance, Mars will show a polar ice cap and any ongoing dust storms. Larger telescopes show dark surface markings which grow and shrink over time – once thought to be vegetation, these are actually just blowing patches of darker dust. A 10” or larger telescope when Mars is at opposition can show actual surface features such as the famed Valles Marineris canyon or Olympus Mons volcano under ideal conditions.

Ceres, Vesta, & Pallas

Best time for viewing:

Almost any

Best magnification:

300x or more

Best aperture:

10” or more

Ceres (also considered to be a dwarf planet), Vesta, and Pallas make up most of the mass in the asteroid belt. All three can be seen with binoculars, and Vesta is sometimes bright enough to see with the naked eye under suburban or dark skies at its peak of magnitude 5.5 (outshining Uranus).

Ceres’ angular size is big enough to resolve it with a 10” or larger telescope as a tiny disk, though its roundness makes it very difficult to distinguish from background stars. Vesta is a bit smaller, but it can get closer to Earth and its ovoid shape is possible to distinguish with a 10” or bigger instrument, very good seeing conditions, and very high magnification of 300x or more, which bad seeing can often prohibit you from using. Pallas will remain a star-like point in any telescope.

Jupiter

Best time for viewing:

Within a few months of opposition, though visible nearly year-round

Best magnification:

60x or more

Best aperture:

6” or more

Upcoming opposition dates

Jupiter of course dazzles with the naked eye under any conditions, being the brightest of the planets besides Venus and occasionally Mars.

Jupiter’s 4 large Galilean moons of course dazzle even in binoculars. A small telescope will show you Jupiter’s cloud bands and the Great Red Spot, while a 6” or larger instrument reveals fine details within Jupiter’s cloud structures and the disks of its moons (along with their shadows when they transit). An 8” or larger telescope can even reveal Galileo Regio, a brown spot on the moon Ganymede, under ideal conditions – though 12” or more makes it easier. A 12” or larger scope will reveal subtle shading on the other large moons – Io’s equator is brighter than its poles, Europa’s disk has the appearance of a dirty snowball, and Callisto looks speckled. Jupiter has many small asteroid-like moons orbiting it, too, but all require huge telescopes and often star charts to find due to their dimness and distance from Jupiter.

Jupiter’s angular size changes quite a bit based on its distance from us; at its absolute furthest it is only 30 arc seconds across and at its best (such as at opposition in 2022) it reaches 50 arc seconds. If Jupiter was a little bigger or closer to us, it would be possible to resolve its disk with sharp eyesight alone.

Saturn

Best time for viewing:

Within a few months of opposition, though visible nearly year-round

Best magnification:

100x or more

Best aperture:

6” or more

Upcoming opposition dates

Saturn’s rings are visible with any telescope, and they’re gradually appearing thinner and dimmer as Saturn approaches its equinox in 2026, during which the rings will appear virtually edge-on and almost vanish from sight before becoming more tilted – and thus brighter – once again. Any telescope with around 3” or greater aperture shows the Cassini Division in the rings, larger telescopes around 6” or so reveal subtle cloud banding, and a 10” or bigger telescope can show another gap, the Encke division, under ideal conditions with a steady atmosphere.

Saturn’s moons are all smaller than Jupiter’s, with the exception of Titan, which is as big as Ganymede or the planet Mercury and can be seen even with binoculars. Saturn has several other moons which can be seen in backyard telescopes: Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, and Hyperion, none of which require a scope bigger than 10” to see.

Uranus

Best time for viewing:

Visible nearly year-round, opposition occurs around mid-November for the foreseeable future

Best magnification:

200x or more

Best aperture:

10” or more

Uranus can be faintly seen with the naked eye under dark skies, though binoculars or a finder scope will show it under light-polluted conditions and 60x magnification or more clearly reveals its turquoise disk. An 8-10” or larger telescope can show faint cloud banding on Uranus, though a 12” or 14” will have better luck provided you have good seeing conditions. 4 moons of Uranus – Ariel, Umbriel, Oberon, and Titania – can be seen with a 10” or larger telescope and all are around magnitude 14 in brightness, though Oberon and Titania are slightly brighter and their distance keeps them out of the worst of Uranus’ glare.

Neptune

Best time for viewing:

Visible nearly year-round, opposition occurs around mid-November for the foreseeable future

Best magnification:

200x or more

Best aperture:

10” or more

Neptune cannot be seen with the naked eye at magnitude 8, though it’s easy to spot with a finder scope or binoculars. A 4” or larger telescope will technically resolve its disk, though a 10” or larger instrument is better for seeing it clearly. Neptune’s moon Triton – a captured dwarf planet larger and more reflective than Pluto – can be seen with a 6” or larger telescope next to the planet under good conditions. Aside from its disk and bluish color, there is little to be seen on Neptune itself due to its tiny angular size.

Pluto

Best time for viewing:

Visible nearly year-round, opposition occurs around late July for the foreseeable future

Best magnification:

50x or more

Best aperture:

10” or more

Pluto, the former ninth planet, is currently located in the constellation Sagittarius and thus faces the unfortunate predicament of being low in the sky for Northern Hemisphere observers. It is also gradually getting further from the Sun; 30 years ago Pluto was at perihelion and reached magnitude 13.5, within range of a 6” telescope under good skies. Today, Pluto is around magnitude 14.5 (more than twice as faint) and it’s generally regarded as a challenge for a 10” telescope; light pollution hasn’t helped and you’ll need fairly dark skies to see it with anything smaller than a 14-16” instrument. Pluto will get down to magnitude 16 when it reaches aphelion about a century from now, putting it out of range of anything under 14-16” of aperture even under dark skies. So don’t delay – look at Pluto as soon as you can, because for most people reading this article it is probably the brightest it will be for the rest of your life. In any telescope Pluto appears as a star-like point, even the Hubble Space Telescope cannot clearly resolve its disk and its large moon Charon is far too close to separate from it (though at magnitude 17.5 it would theoretically be within the range of a 20” or so instrument otherwise).

Makemake & Haumea

Best time for viewing:

Visible nearly year-round, opposition occurs around early-mid April for the foreseeable future

Best magnification:

100x or more

Best aperture:

14” or more

Makemake and Haumea are some of the larger dwarf planets and orbit in the Kuiper Belt at similar distances to Pluto. Both are currently located in the constellation Bootes and are visible with a 16” or larger telescope as magnitude 17.5 star-like points, though a 20” or bigger scope will make it easier and dark skies are required in order to see them with any size instrument.

Eris

Best time for viewing:

Visible nearly year-round, opposition occurs around mid-October for the foreseeable future

Best magnification:

100x or more

Best aperture:

30” or more

Despite being the 9th most massive body orbiting the Sun (beating out Pluto and all the other dwarf planets), the same size as Pluto, and more reflective, Eris is currently just past aphelion, its most distant from the Sun, at 95 astronomical units (95 times as far as the Earth is from the Sun) or nearly triple the distance of Neptune or Pluto from the Sun. As such, it is extremely dim, and at magnitude 18.5, even a 25” telescope is probably insufficient to see it. A 30” or larger telescope is likely necessary for any hope of picking it out even under dark skies. Only a handful of people have ever seen Eris, though it was observed visually with an 82” telescope just months after its discovery in 2006.

Things won’t always be so bad, though. By the end of this century, Eris will peak in brightness above magnitude 18, and thus be within range of a 20” or larger telescope if dark skies still exist. And in the mid-2200s, Eris will be at perihelion, slightly closer to the Sun than Pluto is currently and peaking at magnitude 14.5, within range of a 10” or so instrument.