Which Planets Have Phases Like the Moon?

Did you know that Mercury Venus Mars and Earth all have phases similar to the Moon? And did you know that Saturn has moons too? You might be surprised to know that the Apollo astronauts were able to view Earth’s phases while on the Moon. So how do you find out which planets have phases like the moon? Continue reading to find out! It will be fun to explore these fascinating phenomena in our solar system!


Have your students draw a model of Venus comparing its motion around the Sun with two models: geocentric and heliocentric. Have them compare predictions of their models and discuss the differences. Students can also draw pictures of Venus showing the moon phases in each. This activity can be done over several days. For each cycle have your students draw Venus as a model on a white sheet of paper. If they want they can take photos of Venus with their smart phones.

Because Venus has phases like the moon you can see its brightness change at different times. Ancient astronomers were well aware of Venus’s changes in brightness. The planet is sometimes a dim star but can be the second brightest object in the sky after the Moon. In addition to its changing brightness Venus can cast shadows. Galileo’s discovery of Venus phases was one of the most important moments in the history of astronomy. This discovery shattered the Ptolemaic system and supported Copernicus’ solar system theory.

Galileo first saw Venus’ phases through his telescope and his observations were among the most important in human history. They provided the first observational proof for the Copernican system. Previously the Ptolemaic system had been the prevailing view. But Galileo’s observations confirmed the Copernican theory and led to the discovery of the ‘Dresden Codex.’


When the Earth orbits the sun its heavenly neighbor Mercury will also have phases of its own. The bright streaks visible on Mercury’s surface are called crater rays. When an asteroid or comet strikes the surface the energy released during the impact digs a huge hole and crushes a huge amount of rock. This rock then falls to the planet’s surface where it is more reflective than the larger pieces. This crater ray pattern continues until Mercury’s aphelion. The rays become darker with time due to the atmosphere of space.

Before the discovery of the Earth-Mercury system scientists believed that Mercury rotated on its axis three times per orbit of the Sun. But that was incorrect. Mercury actually rotated only once during an orbit. Mercury’s sidereal day lasts for about 58.7 days while its three-second orbital resonance with the Sun makes it stable. But the truth is Mercury’s two-day-period cycle has no obvious asynchronous motions.

Scientists believe Mercury’s surface was formed by an impact that fractured the planet’s crust and created the Caloris basin which is one of the largest in the Solar System. The Caloris impact caused lava eruptions that left a large concentric ring over two kilometers tall on Mercury’s surface. In addition to the craters Mercury’s surface features many compression folds which are the result of the planet’s interior cooling.


If you’ve ever wondered if Earth has phases look no further than the skies. Not only does Earth orbit the sun but it also has phases of its own. The Moon’s phases are opposite of those of the Earth but we can still observe them. You may even see Earth’s shadow if you look at the moon during its phases. Here’s how to spot them in the sky. This simple explanation will help you identify them when they’re in the sky.

You can also use a flashlight to demonstrate how the phases of the Moon look on the Earth. To demonstrate this simply aim the flashlight at a table hold two balls between them and slowly move them around your head. Your baseball will go through the phases in a similar fashion. Once you’ve done this you can try the same demonstration with the Earth. However the moon will be a bit further away than the earth making it easier to distinguish the two.

The moon is tidally locked to the Earth meaning its orbital period is the same as the Earth’s rotational period. The orbital period of the moon is about a month and the lunar rotational period is about the same as Earth’s. Because the Earth’s rotational period is the same as the Moon’s it will appear to go through phases during a month. In fact the Earth will appear to rotate at its axis for about a month before reversing course.

Saturn’s moons

The Saturn system is home to dozens of moons. The vast deris field that surrounds Saturn has trapped countless smaller satellites. Some of the larger moons have phases that correspond to seasons and astronomers believe they may have different names. While the 82 named moons of Saturn are the most prominent there are still hundreds more to discover. This is why astronomers have been studying these moons for decades.

Saturn’s irregular moons which have radii between six and nine kilometers inclinations between 46 and 59 degrees are classified into three groups. The Inuit Group contains the two newly discovered prograde moons and is named after Inuit mythology. The two other newly announced moons are the retrograde Saturnian moons and have similar inclinations to the previously known retrograde moons. These two moons named Idita and Phoebe were discovered by Italian astronomer G.D. Cassini in 1979. The Norse group contains the two retrograde moons of Saturn which have names that come from Norse mythology.

The inclinations of the midsized Saturn moons are not expected to change much over the entire solar system’s age which makes the observed inclinations of the smallest Saturn moons remarkably similar to the moon’s. They are captured in a subresonance and are subject to tides which damp their eccentricities. Nevertheless this has not prevented the observation of a relatively large number of the Saturn moons which suggest that they have fast tidal evolution.

Ptolemaic system

In the Ptolemaic system the earth sun and other heavenly bodies travel in a circle as if the earth is a fixed point. This is contrary to what we see today when planets appear to have irregular orbits as seen from Earth. Instead these heavenly bodies follow the motions of a rotating spherical disk. Ptolemy’s system had been around for centuries but it was only in the 16th century that it was challenged.

The model he developed later became the basis for the popular understanding of the zodiac. The moon’s phases and epicycles were important concepts in the Ptolemaic era and their explanations helped make the planets easier to understand. This led to the evolution of the equant model of planet motion and the Ptolemaic system. But Ptolemy didn’t end his work with that. He made two great advances on Hipparchus’ work.

As far as the Ptolemaic system goes Venus is never very far from the Sun and it’s not unusual to see it in its crescent phase though its orbit around the Sun might change. The Copernican system on the other hand shows that the moons of planets do not fall behind their heavenly bodies. In the Copernican system Venus exhibits a full set of phases over time.

Copernican system

In the Copernican system of planetary motions planets can either be at the eastern or western quadrature. Superior and inferior planets can never be at the same place at the same time. This is why the Moon is the only object in the solar system that has two phases. The elongation of a planet is proportional to its distance from the Earth. Similarly the planet’s brightness changes during the phases of the moon.

Although the Copernican system removed the Earth as the center of the Solar system it did not remove the need for epicycles. However Copernicus did not question his assumptions about the motion of planetary bodies. Because planets’ orbits are ellipses a system without epicycles would not be able to account for planetary motions on the celestial sphere.

Although the Copernican system has a better explanation for the motion of the planets it is not universally accepted. Galileo himself stated that the Copernican system is difficult to comprehend. Copernicus was a member of the Catholic church which believed in the geocentric solar system and was not supportive of the heliocentric model. His book De Revolutionibus Orbium Coelestium was published in 1633 when he was almost dead. However the introduction to the book refers to the Copernican model as a tool for calculating the motion of planets. This makes Galileo’s model not real.

Despite this fact Copernicus’ work was a controversial one and the book was published at the end of his life. Copernicus feared ridicule from his peers and disfavor from the Church which elevated Aristotle’s ideas to religious dogma. Nevertheless the book triggered a chain of events that led to the development of the Copernican system. Even though the ideas of Copernicus were controversial they were largely ignored for another 100 years.