Eye Astronomy #2: The Planets
By Dale E. Lehman
The sky changes from night to night. Most of these changes are subtle. The moon’s shifting phases and location are the most obvious. (See Eye Astronomy #1.) That’s because it’s the closest object to the Earth. But if you pay attention over time, you can see changes in the positions and even the brightness of the planets.
The word planet means “wanderer.” And the planets do wander through a narrow band of sky. Before the advent of the telescope, only five planets were known: Mercury, Venus, Mars, Jupiter, and Saturn. People with good vision can see Uranus under dark skies, but it’s faint and moves so slowly that it wasn’t recognized as a planet until William Herschel discovered it telescopically in 1781.
Even today, the five naked-eye planets are a great target for eye astronomy. They are among the brightest objects in the sky. Most of them are visible even from light-polluted cities. Once you know a bit about them, you can pick them out with little effort. Here are a few tips to get you started:
Venus is the brightest object in the sky, apart from the Sun and the Moon. Because it’s closer to the Sun than us, it never strays farther than 47 degrees from the Sun, a bit more than halfway from the horizon to the zenith overhead. Sometimes it’s a “morning star,” rising before the Sun, while other times it’s an “evening star,” setting after the Sun. Once you’ve seen Venus, you won’t mistake it for anything else. It’s not just bright. It’s brilliant. Indeed, some people can find it in broad daylight!
Jupiter is the second brightest object in the sky. At its brightest, it’s only slightly dimmer than Venus at its dimmest. Most of the time, there’s a marked difference between the two. Jupiter’s brightness is amazing, given that it’s much farther from the Sun and both Earth than Venus. It’s so bright because it’s huge, with a radius 11 times that of Earth.
Mars is generally dimmer than Jupiter, but it’s still bright, and when closest to Earth can even be a bit brighter than its giant neighbor. Even at its dimmest, it’s brighter than nearly every star, just a bit dimmer than Sirius, the brightest star in the sky. But what really distinguishes Mars from the other planets is its red-orange color. Only a few bright stars have a similar color.
Saturn with its broad rings may be most people’s favorite planet telescopically, but with just your eyes, it’s not too impressive. A bit smaller than Jupiter and much farther away, it normally looks like one of the brighter stars. Its color is a bit yellowish compared to Jupiter. Saturn was the Roman god of time, appropriate for his celestial namesake. Of all the naked-eye planets, Saturn moves slowest, taking 29.4 years to orbit the sun and make one complete circuit around the sky.
Mercury is brighter than Saturn, but it’s the hardest naked-eye planet to observe. It’s never more than 28 degrees from the sun. That means at best you have two hours in the morning or evening to catch it, often less. And it moves fast, completing one orbit in just 88 days. In any given appearance, it’s only visible for 2 to 7 weeks. Keeping so close to the Sun, it’s dimmed—often considerably—by our parent star’s glow. To catch Mercury, you must know when to look and have a clear view of the horizon.
Not all planets are visible at any given time. Sometimes they are behind the sun, or so close to it that they’re lost in its glare. Only Venus is bright enough to cut through daylight. So mostly, you’ll hunt planets in the dark between sunset and sunrise, and what you’ll catch depends on where the planets are in their orbits.
You can find help locating the planets online. One good resource is at EarthSky.org:
https://earthsky.org/astronomy-essentials/visible-planets-tonight-mars-jupiter-venus-saturn-mercury/
You can also ask your friendly neighborhood astronomers at HCAS! We’ll be happy to tell you which planets you can see and where and when to look for them.
