Req 5a — Visible Planets & Phases

Five planets in our solar system are bright enough to see with your naked eyes, and humans have watched them wander among the stars for thousands of years. In fact, the word “planet” comes from the Greek word planetes, meaning “wanderer,” because these objects move against the fixed background of stars.

The Five Visible Planets

These are the planets you can see without any telescope or binoculars:

Mercury — The closest planet to the Sun and the smallest. Because it orbits so close to the Sun, Mercury is always low on the horizon, visible only briefly after sunset or before sunrise. It is the hardest of the five to spot.

Venus — The brightest planet and the third-brightest object in the sky (after the Sun and Moon). Venus is often called the “Morning Star” or “Evening Star” because it appears in the east before sunrise or in the west after sunset. It is unmistakable — far brighter than any star.

Mars — Recognizable by its reddish-orange color, caused by iron oxide (rust) on its surface. Mars varies dramatically in brightness depending on where it is in its orbit relative to Earth. At its closest approach (called “opposition”), it can be almost as bright as Jupiter.

Jupiter — The largest planet in the solar system and the second-brightest planet after Venus. Jupiter appears as a steady, brilliant white point of light. Through binoculars, you can see its four largest moons.

Saturn — The farthest visible planet and the faintest of the five, but still brighter than most stars. Saturn has a warm, golden color. Through a telescope, its famous rings are visible.

Why Some Planets Show Phases

Here is the key concept: only planets that orbit between Earth and the Sun can show a full range of phases like the Moon does. These are called inferior planets, and there are two of them — Mercury and Venus.

Mercury and Venus show phases because their orbits are closer to the Sun than Earth’s orbit. As they move around the Sun, we see them from different angles:

• When Mercury or Venus is on the far side of the Sun from us, we see its full sunlit face (like a full Moon), but it appears small because it is far away.
• When it is between us and the Sun, the sunlit side faces away from us (like a new Moon), and it is invisible.
• At positions in between, we see crescent and half phases, just like the Moon.

Galileo’s discovery of Venus’s phases in 1610 was revolutionary — it proved that Venus orbits the Sun, not Earth, and helped confirm the Sun-centered model of the solar system.

Mars, Jupiter, and Saturn do NOT show a full range of phases. These are superior planets — they orbit farther from the Sun than Earth does. Because we are always between them and the Sun (or close to it), we always see most of their sunlit side. Mars can show a slight “gibbous” phase (slightly less than full), but it never appears as a crescent. Jupiter and Saturn are so far away that they always look essentially full from our perspective.

A Simple Way to Remember

Think of it like standing in a room with a lamp (the Sun) in the center:

• If your friend (Venus) is between you and the lamp, you see their dark silhouette — a “new” phase.
• If your friend moves to the side, the lamp illuminates half their face — a “quarter” phase.
• If your friend is on the far side of the lamp, you see their fully lit face — a “full” phase.

Now imagine a friend (Jupiter) who is always on the far side of the room from the lamp. No matter where they stand, the lamp always illuminates most of their face from your viewpoint. You never see their dark side because the lamp is always between you and them.

Next, let’s find out when you can actually see these planets in the sky.