The Phases of Venus

When Galileo first looked at the heavens with his telescope, little did he expect to show that the heliocentric model was the more correct model than the geocentric model.  As mentioned in the previous post, there were many things that he was able to show just by looking at the sky.  Here, we will focus on looking at the phases of Venus and why the heliocentric model is the correct model of the solar system.
In the geocentric model, Venus orbits the Earth on an epicycle, and must be always close to the Sun in the sky (this will be explained in the next post). 

Copernican Revolution



So what does this show?  It shows that from Earth, we never see Venus "full" or its face being totally illuminated by the Sun.  What we see is Venus either new or in crescent phase.

When Galileo looked at Venus, he observed that Venus also had quarter phases and gibbous phases and assumed (based on his observations), that if we could see Venus, it would be full. 

 

Copernican Revolution

 

Looking at the above image, we see a couple of things.  When Venus is closest to the Earth, it is in what is called inferior conjunction*. 

 

*Conjunction is a term when a planet and the Sun are in the same direction in the sky. If the planet and the Sun are in opposite directions (i.e. 180° away from each other), they are said to be in opposition.  Obviously, based on Venus' location in the solar system, it can never be in oppostion.

At inferior conjunction (E in the above picture), Venus is considered to be in its new phase (as seen from Earth).  The unluminated portion of Venus' surface is facing Earth, much like during the new Moon, we see the darkened face of the Moon.  Venus is between us and the Sun.

At superior conjuntion (A in the above picture), Venus is considered to be in full phase (as seen from Earth), if we could see Venus.  As shown, the Sun is between Earth and Venus.

There are two other locations on Venus' orbit that will be discussed in the next post: greatest eastern elongation and greatest western elongation.  These are the points in the orbit where Venus is the farthest east from the Sun or the farthest west from the Sun as seen from Earth.  These are the "quarter" phases of Venus.

Galileo was able see Venus go through phases as he observed it just before sunrise and just after sunset.  By watching how much of Venus face was illuminated and recording what he saw, he could show that Copernicus and Kepler were right.  Venus did not orbit the Earth; but instead, orbited the Sun.  To the same degree, if you could observe Mercury, you will be able to see Mercury go through the same phases as Venus, and also show that Mercury orbits the Sun.

The Phases of the Moon

We are going to take a break from talking about Venus to explain about the phases of the Moon.  This background is necessary when we get into the next post about Venus.

The Moon goes through basically eight phases as it orbits the Earth.  These phases are a result of the Moon's alignment with both the Earth and the Sun.  Before we go into the descriptions of the phases, we should define two terms:

The sidereal lunar cycle, or "sidereal month" is how long it takes the Earth, Moon, and a background star to line up.  This period last about 27.3 days.  After another 2.2 days, the Earth, Moon, and Sun are lined up in what is called the lunar month, or synodic month.  This period is approximately 29.5 days.  The difference between these periods is because as the Moon orbits the Earth, the Earth is also orbiting the Sun.

The first phase during the lunar month is called the "New Moon".  Basically, it is when the Moon is between the Earth and the Sun.  The unilluminated face of the Moon is pointed towards the Earth.  However, the new Moon is not completely dark.  Earthshine, reflection of sunlight off the Earth's surface can illuminate the surface of the Moon.

Farmer's Almanac Online

 

The next phase is the Waxing Crescent phase.  Waxing in astronomy means that something is increasing in size.  As the Moon moves around the Sun, a sliver of the Moon on the right limb is illuminated by the Sun as seen from the Earth.  What we see is the crescent shape of the Moon.





 

Farmer's Almanac Online

The Moon is said to be aging as it goes from New Moon to Waxing Crescent.  About a week after the New Moon, the Moon reaches the First Quarter phase because it has reached the end of one quarter of the lunar cycle.  The entire right half of the Moon is illuminated as seen from the Earth.

10 Minute Astronomy



 

The next phase is called Waxing Gibbous.  The illuminated surface of the moon crosses the center of the moon and spills over into the left half of the Moon.  The illuminated portion of the Moon resembles an oval with pointed ends.

Farmer's Almanac Online



Halfway through the lunar cycle, we hit the Full Moon. No, werewolves do not appear during the Full Moon.  People do not get crazy during the Full Moon, though it may appear to be so.  The Full Moon is the phase where the Earth is between the Sun and the Moon and from Earth we see the entire face of the Moon illuminated.

Farmer's Almanac Online

After the full Moon passes, the illuminated portion of the Moon begins to decrease, or wane.  The next phase is the Waning Gibbous phase.  The right limb has a darkened crescent shape, and the left half and a portion of the right half of the Moon are illuminated, opposite the Waxing Crescent phase.



Farmer's Almanac Online

After three weeks in the lunar cycle, the Moon reaches the Last Quarter or sometimes called Third Quarter.  It is called this because either the Moon is at the beginning of the last week of the lunar cycle or the end of the third week of the lunar cycle.

Astropixels



In the final week before the Moon is "reborn" into the New Moon, the Moon goes through the Waning Crescent phase.  Only a sliver on the left limb of the Moon is illuminated. 

Farmer's Almanac Online

 

Here is an image showing the relative position of the phases with respect to the Earth and the Sun.





From Swinburne Astronomy Online