How to Tell the Difference Between Planets and Stars

Observing Physical Differences

Check if the object twinkles. One of the easiest ways to distinguish between stars and planets in the night sky is by looking to see if the object twinkles or shimmers. This can usually be detected with the naked eye if you have a clear view of the sky and if you watch the sky for long enough. Stars twinkle and shimmer - hence, the song "Twinkle, Twinkle Little Star." Planets do not twinkle. They remain constant in their brightness and their overall appearance in the night sky. If viewed through a telescope, planets may appear to "wiggle" along the edges. Any object that blinks, twinkles, or shimmers is most likely a star. However, it could also be an aircraft if it is moving quickly across the night sky.

Note whether the object rises and sets. Celestial objects are not fixed in the night sky. All celestial bodies move, but how those bodies move can be a good indication of whether they're stars or planets. Planets rise in the East and set in the West. They tend to follow a similar celestial path across our sky as the sun and moon. Stars move around in the night sky, but they do not rise or set. Instead, they orbit in a circular pattern around Polaris (the North Star). If the celestial object you see appears to move in a more-or-less straight line across the night sky, it is most likely a planet. Satellites also move across the night sky, but they do so much more quickly than planets. A planet may take hours or even weeks to cross the night sky, whereas a satellite can cross overhead in a matter of minutes.

Identify the ecliptic. Planets are always found along an imaginary belt across the night sky called the ecliptic. This belt is not actually a visible object, but careful observation will help you find the location where celestial objects are congregated. While stars may also appear along this invisible belt, they should be distinguishable by their shimmering appearance. Of the celestial bodies along the ecliptic, Mercury, Venus, Mars, Jupiter, and Saturn are significantly brighter than the surrounding stars. This is due to their proximity to the sun since their "brightness" is just reflected sunlight. The easiest way to find the ecliptic is to note the location and trajectory of the sun and moon in the sky relative to your location on the Earth. The sun's path across our sky is very close to the path of planets along the ecliptic.

Observe the color. Not all planets are colorful. However, many of the most prominent planets in our night sky appear to have some type of coloration. This can help differentiate the planets from stars. While some people with exceptionally good vision may be able to detect subtle coloration, that coloration usually falls within the blueish-white to yellowish-white range. For most people, stars appear white to the naked eye. Mercury is typically gray or somewhat brownish in color. Venus appears pale yellow. Mars usually appears somewhere between pale pink and bright red. This is affected by the relative brightness or dimness of Mars, which changes on a two-year cycle. Jupiter appears orange with white bands. Saturn typically appears pale gold in color. Uranus and Neptune appear pale blue. However, they are not usually visible to the naked eye.

Compare the relative brightness. While planets and stars both light up the night sky, planets typically appear much brighter than many stars. Astronomers measure the relative brightness of celestial objects using the astronomical magnitude scale, with most of the planets falling in the range of objects that are easily visible to the naked eye. Planets reflect the bright light of our solar system's sun, which is relatively close to the earth. Stars, by contrast, emit their own light. While some stars may be much brighter and larger than our sun, these stars are much farther from the earth than the planets in our solar system. Because of this, the planets (which reflect our sun's light) typically appear brighter from the earth.

Observing Celestial Bodies

Bring star charts and planetary guides. Whether you have poor night vision or are simply confused about the location of certain celestial bodies, a chart or guide can help you determine where to look. You can purchase star charts and planetary guides from a bookstore, print out free guides from the internet, or download a star/planetary guide app on your smartphone. Remember that star charts are usually only valid for a limited period of time (typically about one month). That's because the position of stars in the sky changes over time as the earth continues along its orbit. If you consult a star chart or planetary guide out in the field, make sure you use a dim red flashlight. These flashlights are designed to provide light without affecting your eyes' ability to adjust to the darkness.

Get a good telescope or binoculars. If stargazing with the naked eye isn't getting you enough celestial body views, you may want to consider using a telescope or binoculars. These instruments can help assist your view by magnifying the area you look at. This can make visible objects clearer and can even bring objects invisible to the naked eye into your view. Some experts recommend getting familiar with celestial bodies using the naked eye, then trying binoculars, and finally moving up to using a telescope. This can help you get more acclimated to the visible bodies and their place in the night sky. Compare telescopes and binoculars online before you invest in one. Read the reviews written by people who have owned a given model by searching online for the model you're interested in.

Visit a dark-sky site. Light pollution from urban areas can drastically limit your ability to see celestial bodies in the night sky. To really maximize visibility, you may want to consider visiting a dark-sky site. These designated sites have been identified by the International Dark-Sky Association (IDA) as places worth protecting against encroaching light pollution and urban development. Common dark-sky sites include state and national parks, though other dark-sky sites are surrounded by well-lit, well-developed regions. Check the IDA website to find a dark-sky site near you.

Identifying Visibility-Limiting Factors

Check whether an occultation is scheduled. An occultation is when the moon passes between the Earth and a given star or planet, obstructing that celestial body's visibility. These obstructions happen somewhat regularly and can easily be planned around since their occurrence is predictable. Occultations may be visible from some locations on Earth and not from others. Check in advance to see whether an occultation is scheduled and whether your visibility will be significantly affected. You can find out about planned occultations by searching online or by consulting an astronomy guide. The International Occultation Timing Association publishes their predictions online for free.

Identify the moon's phase. Light reflected off the moon can inhibit your ability to see stars and planets. If it is close to a full moon, you may have a hard time observing celestial bodies. For this reason, it's best to check on the current phase of the moon before venturing out to observe the night sky. If you aren't sure about the moon's current phase, you can consult an online guide to the moon's phases for free. The U.S. Navy's website allows you to check moon phases by date as far in advance as the year 2100.

Find the right conditions. Knowing how to distinguish between stars and planets will only get you so far if the night sky is not very visible. Your ability to see celestial bodies can be limited by a number of factors, both man-made and naturally-occurring. Light pollution is one of the biggest limiting factors of night sky visibility. If you live in or near a metropolitan area, you will probably need to travel to a more rural area to increase visibility. Cloud cover and significant snow cover can both affect night sky visibility. If it's very cloudy or if the ground is significantly covered in snow, you may have a hard time seeing celestial bodies in the sky.

Avoid other limiting factors. There are many other factors can also affect night sky visibility, including some that you might be inflicting upon yourself. For example, your level of alcohol consumption, your nicotine consumption, and your pupil dilation at the time of viewing can all affect your ability to see celestial bodies. These factors all affect your eyes' ability to adjust to the dark and identify stars and planets in the night sky.