Star or planet, how can you tell?
By PAUL DERRICK
Have you ever seen a bright star and wondered if that “star” might be a planet? Planets and stars are different types of objects, yet they look alike to the naked eye. Indeed, in the word planet derives from the Greek word “planēs” which means wanderer, thus planets were considered wandering stars by our ancestors.
So, without using a telescope, how can one tell which is which? One way, of course, it to watch any suspicious object night after night to see if it moves, or wanders, relative to the surrounding stars. But that can take many nights as planets move slowly, and the more distant ones have to be watched for weeks, especially when there are no nearby stars.
For casual observers, there’s no foolproof way to pick out the planets from the myriad of stars, but some pointers can help. First, there are only five naked-eye planets – Mercury, Venus, Mars, Jupiter, and Saturn – and they are usually brighter than the vast majority of naked-eye stars. (Technically, Uranus can be seen naked-eye, but it’s at the limit of visibility, and seeing it requires good eyes and very dark skies.)
Start by noticing the brightness of the object in question. Venus and Jupiter always outshine all the stars, and the other three are usually brighter than all but the brightest stars. So brightness is the first clue.
Second, perhaps you’ve heard that “stars twinkle and planets don’t” – and this tends to be true, although the difference can be subtle and varying. Twinkling (called scintillation) is caused by turbulence in Earth’s atmosphere, so that greater air instability produces more twinkling while steadier air makes for less. So, when the stars are twinkling noticeably, look for any brighter objects that aren’t.
Third – and this is quite helpful – look in the right part of the sky. Planets orbit the Sun on nearly the same plane, so they all closely follow the ecliptic – the Sun’s path across our sky. And since the Moon orbits the Earth on nearly this same plane, it too travels close to the ecliptic.
Like the Sun and Moon, the planets rise in the east and set in the west, owing to the Earth’s west-to-east rotation on its axis. Facing south with your arms outstretched slightly more than 180 degrees, any visible planets will be in front of your arms – never behind you or even straight overhead. All that are above the horizon will be somewhere along the great ecliptic arc beginning in the east (your left), rising and tilting somewhat to the south, and ending in the west (your right).
There are two additional things to know about the ecliptic. The season and time of night affect the ecliptic’s exact rising and setting points, making them sometimes a little left or right of due east and west. And, the amount of southerly tilt varies so that sometimes the ecliptic is tilted more than half way down toward the southern horizon while at other times it reaches nearly straight up. Still, knowing the approximate path is useful in identifying planets.
The inner planets, Venus and Mercury, orbit nearer the Sun inside Earth’s orbit. Thus, when they are not hidden in the Sun’s glare, they are seen only in the evening soon after sunset low in the west or in the morning soon before sunrise low in the east – never real high in the sky and never deep into the night. When visible, Mercury, being so near the Sun, is rarely seen after twilight.
The outer planets, Mars, Jupiter and Saturn, when not passing behind the Sun, might rise or set – and thus may be seen – any time of the night. When they are on the opposite side of Earth from the Sun – a position called opposition – we are at our nearest to them, making them appear at their brightest and largest.
Currently there are great opportunities for identifying four planets among the stars as Saturn and Mars are visible in the evening while Venus and Jupiter are up in the morning.
Soon after dark Saturn and Mars are less than half way up in the southwest. Saturn is currently less than 5 degrees (one-half fist-width held at arm’s length) to the upper right of the star Spica. Almost equally bright, Spica is twinkly white while Saturn shows a steady creamy tint. About 20 degrees (two fist-widths) to their lower right, Mars has a reddish tint with no notably bright stars nearby. Mars is gradually approaching Saturn and Spica, and will pass between them in mid-August.
In the morning before dawn, Venus and Jupiter are very low, rising in the east. “Morning star” Venus vastly outshines all the real stars as does Jupiter, although the king of the planets can’t hold a candle to his queen. Venus is near the reddish star Aldebaran, passing within two moonwidths July 8-10, but 1st-magnitude Aldebaran, 14th brightest of all the stars, pales in comparison to Venus. Jupiter, 5 degrees above, is nearly half way between Venus and the lovely Pleiades star cluster. Venus and Jupiter are drifting further apart each morning.
Here’s hoping these pointers help you learn to pick out our brighter solar system neighbors from among the seemingly countless background stars.
Paul Derrick is an amateur astronomer who lives in Waco. Stargazer appears twice monthly. Paul’s website is www.stargazerpaul.com, and contains an archive of past Stargazer columns, a schedule of his upcoming programs, star parties and classes, and other basic stargazing information. Contact him at email@example.com or 254-723-6346.
Story filed under: Big Bend Blog