Main Page

Welcome to the explain xkcd wiki! We currently have 12 comic explanations. Come and add yours!

Latest comic

edit this explanation!

Planets and Bright Stars

|< < Prev #3219 (March 13, 2026)

Title text: An old astronomer trick for distinguishing the Sun from other stars is to take multiple photos a few minutes apart and overlay them, making the Sun stand out due to its high proper motion.

|< < Prev Random Explanation

Explanation

This comic shows an identification chart for some of the planets and bright stars visible at night from Earth. Bright shiny objects in the sky are often confused with each other by people without astronomical experience. This the chart is supposed to make identification easier by placing them adjacent to one another to easily see the differences. The joke is that all 12 dots are nearly identical, making the chart useless.

The real way of distinguishing these objects is by their location in the sky. Stars can be found by using constellations, which are an apparent pattern of bright stars that make different regions of the sky distinguishable from one another. The planets can be distinguished by not belonging to the constellations, and further differentiated by their color, brightness, and movement relative to the stars (on the scale of weeks or months).

The stars and planets do indeed look similar to one another in reality, but they are not identical. Some of them (in particular, the star Betelgeuse and the planet Mars) have a distinct reddish color, which can be seen in good conditions. The brightness is also different, and it can serve as a guide, but it's difficult to precisely judge brightness by eye, and the planets don't have a constant brightness over time. The differences are actually visible in the comic to a degree - e.g., the spots for Venus and Jupiter are slightly larger than the others - but they're subtle enough to not recognize at first glance.

Each 'object' also has a color, albeit extremely desaturated (very nearly white). If deliberately exaggerated, the comic's planets and suns are all notably non-white, as can bee seen in the picture in the trivia section below..

• Planets:
  • Venus: the yellowy-orange hue of its cloud layers.
  • Mars: the red of its surface (given more muted saturation, in the comic, for the joke to work?).
  • Jupiter: the general orange hue of its combined cloud layers.
  • Saturn: a more 'peachy' orange of its clouds (no obvious hint of its ring system).
  • Mercury: a yellow surface (not typically noted, in true-hue images, perhaps artistic licence from its proximity to the Sun).
• Stars
  • Sirius (binary system, primarily a main sequence A-type star): light blue.
  • Procyon (subgiant F-type star): more light green, or yellowy-blue/cyan.
  • Antares (M-type star, red supergiant): orange.
  • Altair (A-type, main sequence): light blue.
  • Betelgeuse (M-type, red supergiant): relatively dark red (usually visible as such in real eyes-only observations).
  • Vega (A-type, main sequence): light blue.
  • Polaris (ternary star-system, dominant member being an F-type yellow supergiant): extremely unsaturated cyan.

Using even a small telescope would make it easy to distinguish the planets by their brightness, size, and surface features. Additionally, using a spectroscope would allow for a measurement of the star's spectrum, which coupled with its brightness would allow an astronomer to distinguish the mentioned stars.

The title text suggests a "trick" for recognizing the Sun among other stars, suggesting measuring its proper motion (a measure of the change in apparent position against the more distant 'fixed' background of stars, as an angular rate to specify some angle per time) by overlaying several images, a similar principle to the blink comparator. This does indeed differentiate it from other stars, but there are much easier methods, such as its extreme brightness and large angular size.^{[citation needed]} A disadvantage of this method is that it distinguishes the Sun from other stars, but it cannot distinguish the Sun from planets. It is also completely unnecessary, except during a solar eclipse, because stars are not usually visible during the day, when the Sun is out. Additionally, "proper motion" is a term usually not used for the Sun.

Transcript

[A 4x3 grid of white dots are displayed on a black background. Above them there is caption in white text. Below each white dot there is a label in white. The dots are almost identical slightly fuzzy and white. There may be a slight color hue to some of the dots, but it is not clearly visible.]

Planets and Bright Stars identification chart

Venus

Mars

Jupiter

Saturn

Mercury

Sirius

Procyon

Antares

Altair

Betelgeuse

Vega

Polaris

Trivia

• User:Rumbling7145 made this picture enhancing the colors of the dots in the original comic:

New here?

Feel free to sign up for an account and contribute to the explain xkcd wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

• If you're new to wikis like this, take a look at these help pages describing how to navigate the wiki, and how to edit pages.

• Discussion about various parts of the wiki is going on at Explain XKCD:Community portal. Share your 2¢!

• List of all comics contains a complete table of all xkcd comics so far and the corresponding explanations. The red links (like this) are missing explanations. Feel free to help out by creating them!

Rules

Don't be a jerk. There are a lot of comics that don't have set in stone explanations, feel free to put multiple interpretations in the wiki page for each comic.

If you want to talk about a specific comic, use its discussion page.

Please only submit material directly related to—and helping everyone better understand—xkcd... and of course only submit material that can legally be posted (and freely edited.) Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users posting such are at risk of being blocked.

If you need assistance from an admin, feel free to leave a message on their personal discussion page. The list of admins is here.

Logo

Explain xkcd logo courtesy of User:Alek2407.