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.

Galileo Galilei

Galileo Galilei was a famous Italian mathematician and astronomer.  He is well-known for being charged with heresy by the Catholic Church in Italy for teaching and promoting the heliocentric model of the solar system.  He was never officially declared a heretic, but he spent the rest of his life under house arrest.  He was not forgiven for this teaching until 1992 by Pope John Paul II.

Why did Galileo believe that the heliocentric model was correct and not the geocentric model, which had been the official Church canon for centuries?  In one word: telescopes.  He used his telescope to look to the skies and observe some things that he saw.  Based on his observations, he concluded that Copernicus and Kepler were right and that Ptolemy was wrong.

Some of his observations were:

1. Jupiter had moons going around it.  It had always been argued that the only object that things could orbit around was the Earth.  Jupiter had four moons that countered this.  I'll discuss the Galilean moons later.
2. The Moon had a bumpy surface.  The Church believed, as did scientists up to this point, that all celestial objects were perfect spheres.  By observing craters and mountains on the Moon, Galileo showed that this was incorrect.
3. He found sunspots on the Sun.  As mentioned in #2, the Sun was thought to be perfect and had no blemishes.  This was incorrect.
4. The milky strip across the night sky was found to contain many stars.  For that matter, the stars on the "fixed sphere" were found to not be fixed, but moved with respect to other stars.  This will be addressed in a future post.
5. Venus experienced phases.  Based on the geocentric model, Venus always had its darkened face towards the Earth, but when Galileo looked at Venus, he discovered that this was not true.  The next post will go into more detail about this and help explain why the phases of Venus could explain why Venus goes around the Sun and not the Earth.

Heliocentric Model

In the 1500s, Nicolaus Copernicus developed what is now known as the heliocentric model of the solar system.  He took the idea of the Ptolemaic model of the solar system with the Earth at the center and realized how convoluted it was and introduced the idea that the Earth and the planets all orbited the Sun.  The only exception to this was the Moon, which orbits the Earth.

This was a revolutionary idea.  For more than 1500 years, the geocentric model was accepted as fact, and the Catholic Church refused to budge, even calling the heliocentric model heresy.  Ironically, Copernicus was a canon of the Catholic Church who had vowed to stay celibate.  His work, the Commentariolus, was a short 40 page essay outlining his ideas.

1. Celestial bodies do not all revolve around a single point, i.e. the Moon orbits around one point, the Earth and the planets around another point
2. The Moon orbits around the Center of Earth
3. The Earth and planets orbit around the Center of the Sun
4. Parallax to stars cannot be observed because the Earth-Sun Distance is so much smaller than the distance to the stars. In reality, parallax is so small that they couldn't be measured until almost 400 years later.
5. The stars don't move.  The motion we see in the stars is due to Earth's motion.
6. Earth moves around the Sun, which causes the apparent annual motion of the Sun.  Earth rotation causes the apparent motion of the Sun in the sky over the course of a day.
7. Earth's orbital motion around the Sun causes the retrograde motion we see in the planets, i.e. why the planets seem to reverse direction and move east to west in the sky relative to the stars rather than west to east.

However, his model did not get rid of a couple of things.  He still felt that the orbits were circular and because of fluctuations in the observation of the planets, he had to keep epicycles.

Bucknell

 

It wasn't until Johannes Kepler came along that a more modern heliocentric model arrived.  Johannes Kepler was an apprentice of Tycho Brahe, who was a strange fellow and may be featured in a later post, and was able to use the thorough and accurate observations made by Brahe to come up with his three laws of planetary motion.

 

1. Planets have elliptical orbits, i.e. circular or oval and the Sun is at one focus.

   

   Hankwang

    
2. A line connection the Earth and the Sun will sweep out equal areas in equal times.  This was discussed earlier here.
   
   
   
   
   
3. Shashi M. Kanbur, Professor of Physics and Earth Science, SUNY Oswego
4. The cube of a planet's semi-major axis distance equals the square of the planet's orbital period, as long as they are measured relative to Earth.  The distance is measured in astronomical units, AU, which is the average distance from the Earth to the Sun. The period is measured in Earth year's.  This was actually proved by Isaac Newton with calculus and physics.
   
   
   
   
   

 

How do we use this to explain retrograde motion of the planets?

Saint Mary's University, Canada

As the Earth overtakes Mars because it is moving faster, Mars "appears" to be moving backwards.  As explained here, it is like a faster car overtaking a slower car on the highway. The slower car appears to be moving backwards with respect to someone in the faster car.

 

I think this post might raise some questions, and if you do, please leave a comment.  I will try to explain them in a future post.

 

Geocentric Model

The geocentric model of the solar system is a model that has the Earth at the center of the solar system and all the planets, Moon, and Sun orbiting around the Earth.

This model made sense to the ancients who could only see the sky from the ground.  Imagine if you didn't know aby better, it looks as if the Sun, the planets, and the Moon all orbited around us.  We know that only the Moon goes around the Earth. But to an ancient, it would make sense that the Sun and planets went around the Earth.  This is why they created a model called the geocentric model.

 

Let's define some terms:

Deferent - the actual orbital path of the planet, Sun, or Moon around the Earth. Everything moves on these orbits counterclockwise looking down on the North Pole of the Earth.

Epicycles - the smaller circles that are centered on a point on the deferent.  These are used for the planets to help explain retrograde motion in the sky.  Without these, retrograde would be difficult to explain.

There is also a line connecting Mercury, Venus, and the Sun so that they stay in relatively the same location in the sky.  This will be explained later.

Lastly, the Earth is offset from the center to account for the changes in size of the Moon and planets as they "orbited" the Earth.

The most famous geocentric model was produced by Ptolemy in the 2nd century.  This model persisted for 1500 years until Copernicus and later Kepler and Galileo were able to prove heliocentric model, which will be our next post.

Both images above are from ScienceU.com.

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

 



Venus' Atmosphere

The atmosphere of Venus is one of the thickest atmospheres in the Solar System, and is by far, the thickest of the terrestrial planets.  It is mainly carbon dioxide (CO2) with 96.5% of the atmosphere made up of CO2.  It has about 3.5% nitrogren (N2) and trace other elements with sulfur compounds beign a major portion.  Compare this to the Earth with 78% N2, 21% O2, and trace other gases (argon being the chief among those gases).

The atmosphere is so thick that the atmospheric pressure at the surface (what we would call sea level on Earth) is 92 times that of Earth.  A cubic meter of air on Earth has a mass of about 1.2 kilograms, or weighs about 10 pounds.  On Venus, the same volume of air has a mass of 67 kilograms, or weighs 600 pounds on Venus (on Earth, that volume of air would weigh 670 pounds).  This weigh is so heavy, that its atmosphere at the surface would squash you flat and kill you, if the oppressive heat didn't get you first.

Besides being oppressive, the heat is the most impressive thing about Venus' atmosphere.  Despite being farther from the Sun than Mercury, its surface temperature is hotter.  Because Mercury has such a thin, if lacking, atmosphere, it does not retain heat well.  With Venus thick atmosphere composed of mostly carbon dioxide, the atmosphere does a great job of retaining heat reflected and emitted by the surface of the planet.  Carbon dioxide a really good job of preventing infrared radiation from escaping into space which in turn heats up the atmosphere.  This lead to a runaway greenhouse effect which increases the heat on Venus' surface.  On Venus, surface temperatures can reach 462°C (864°F) where on Mercury, in sunlight, reaches "only" 420°C (788°F).  Mercury does drop below freezing on the side facing away from the Sun at -220°C (-364°F) because the lack of an atmosphere.

At the same time, it is nearly impossible to see the surface of Venus without some help.  The intense cloud cover does not allow visible light to escape.  On Earth, our clouds are made of water vapor and droplets.  Venus' clouds are hydrogen sulfide and sulfuric acid.  Not easy material for visible light to traverse.  These clouds allow 50% of the visible light to come through and heat the ground, leading to the reflection and emission of infrared light, while the other 50% is reflected into space.  What we see when we look at Venus is the cloud cover.

To see Venus itself, we use radio waves which have long enough wavelengths to travel through the clouds.  The reflected radio waves can then be detected and map the surface.  This was what the spacecraft Magellan did to show us the planetary features.  Any images of the surface of Venus are all false color.

The Surface of Venus

Venus was bombarded by meteoroids in the past much like Earth was.  We know this by looking at Mercury, Earth, the Moon, and Mars.  They were all pelted continuously by asteroids and comets that left behind craters when they impacted the surfaces.  But why don't we see many craters on Venus?

The main reason is volcanism.  There are three objects in the solar system with active volcanism: Earth, Venus, and Jupiter's moon Io.  There is evidence that both Mars and minor planet Vesta had volcanism in the past, but now are dormant bodies.  On Venus, the volcanism is ongoing; this is evidenced by the amount of sulfur compounds in the atmosphere.  The volcanism is continually reshaping the surface of Venus, much like it does on Earth.  On Venus, the volcanism can lead to interesting features.

The first are flat-topped mountains called farra.  On Earth, volcanoes form calderas and spew forth lava which can make the volcano taller, or in most cases, widen the base of the volcano.  Farra, on the other hand, are tall (100 m to 1000 m high), but very wide (20 km to 50 km).

 

 

There are also features called novae.  They get the name from the star-like appearance they take on.  The nova are fractures emanating radially from a central region.

 

There are coronae.  These are concentric circles centered on a central depression.

 

 

 

Finally, there are arachnoids.  These are spiderweb like features that combine the look of novae and coronae.

 

 

Venus also has two main landmasses, similar to continents on Earth.  They are Ishtar Terra, named after the Babylonian goddess of love and Aphrodite Terra, after the Greek goddess of love.  It is on Ishtar Terra where we find the the taller mountain on Venus, Maxwell Montes, 11 km tall (above average elevation, what we refer to as sea level).  Note that Mount Everest is only 8.85 km tall.  The weaker gravity on Venus allowed Maxwell Montes to grow taller.  We will see the largest mountain in the solar system on Mars, with weaker gravity than both Earth and Venus.  Maxwell Montes is named after the physicist James Clerk Maxwell who is well known for his laws of electricity and magnetism.  Using his four equations, he was able to predict the existence of radio waves, electromagnetic radiation with the longest wavelengths.  Using radio waves, scientists were able to determine what the surface of Venus looks like.  Maxwell Montes is one of the four features on Venus not named after a female; mythological or historical.  The other three features are:

• Alpha Regio: a tessera in the southern hemisphere
  
• Beta Regio: a volcanic rise in the northern hemisphere
  
• Ovda Regio: the western portion of Aphrodite Terrra that has a complex surface dominated by a large caldera in the far west
  

All of these features were formed by volcanism.

Next time, we will learn about the atmosphere on Venus.