The Morning Star and The Evening Star

Have you ever heard the terms "Morning Star" or "Evening Star"?  These terms are actually misnomers.  Neither object is actually a star.  They both refer to planets, either Venus or Mercury.  Today, we will answer why they are referred to either of these terms.

The ancients knew that both Mercury and Venus were close to the Sun.  They could only see them either in the morning or the evening.  This is how they got the name of Morning Star or Evening Star.  When they are the Morning Star, they rise just before the Sun and therefore, are only seen in the eastern sky.  When they are the Evening Star, they set just after the Sun and therefore, are only seen in the western sky.

If the ancients believed that everything in the solar system orbited around the Earth, how did they explain how Venus and Mercury stayed close to the Sun?  In the Ptolemaic model, the epicycles for both Mercury and Venus were connected to the Sun by a line.

Copernican Revolution



 

This line was needed in the model to keep Venus and Mercury close to the Sun in the sky.  Otherwise, in this model, Venus or Mercury and the Sun could be at opposition, which we know is not true.

 

When the Copernican Revolution occured and the heliocentric model came to prominence, the line was no longer needed.  Since Mercury and Venus orbit the Sun and in the model, are closer to the Sun than the Earth, their proximity to the Sun in the sky is easier to explain.

Copernican Revolution



We discussed the Phases of Venus in our last post and mentioned the greatest eastern elongation and the greatest western elongation.  For Mercury and Venus, we use these terms to tell us the point in Mercury's and Venus' orbits when they appear the farthest from the Sun in the sky depending on their position relative to the Sun.

Greatest eastern elongation occurs when Venus (or Mercury) is the farthest east of the Sun.  When either planet is at greatest eastern elongation, the planet is only visible from Earth just after sunset, which means we only see it when it’s in the western sky.  We call this the Evening Star.

Greatest western elongation, however, occurs when Venus (or Mercury) is the farthest west of the Sun.  When either planet is at greatest western elongation, the planet is only visible from Earth just before sunrise, which means we only see it when it’s in the eastern sky.  We call this the Morning Star.

Remember that the terms refer to the planet’s position with respect to the Sun in the sky, and not its location in the sky when it is visible.

By comparing the position of Mercury or Venus to the Sun, we can determing the angle between the planet and the Sun at either greatest elongation.  Venus cannot be any more than 47.8° from the Sun in the sky. At greatest western elongation, Venus rises about 2.5 hours before the Sun and at greatest eastern elongation, Venus sets about 2.5 hours after the Sun.  Mercury cannot be any more than 27.8° from the Sun in the sky.  At greatest western elongation, Mercury rises about 1.5 hours before the Sun and at greatest eastern elongation, Mercury sets about 1.5 hours after the Sun. 

Note that these elongations occur when the planet is at aphelion.  If either planet is at perihelion, the angle is smaller.  For Mercury, greatest elongation at perihelion is only 18° and is visible an hour before or after the Sun rises or sets.  For Venus, greatest elongation at perihelion is 45° which doesn't change its appearance before or after sunrise or sunset by much.

The reason for the huge difference for Mercury's elongations is its relatively elliptical orbit.  Venus' orbit is closer to circular so the angles are much closer.

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.

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.

 

Venus

 

Venus, Earth's twin, is the second planet from the Sun.  In terms of size, it is the sixth largest planet, larger than Mercury and Mars.  It is 95% the size of Earth and 81.5% the mass of Earth.  If you were to stand on Venus, besides dying, you would weight 90% of what you do on Earth.

 

Venus has a relatively young surface which indicates that it is geologically active.  It has active volcanoes, as evidenced by the amount of sulfur in its atmospher.

 

Venus has two contintents, named after two goddesses of love, the Greek Aphrodite and the Babylonia Ishtar.  It has a large mountian, Maxwell Montes, one of the few features not named after historical or mythological females.

Venus also has a much thicker atmosphere than Earth, even though it is smaller than Earth, as mentioned above.  This thick atmosphere actually makes Venus the hottest planet, hotter than Mercury.

 

When Galileo Galilei first turned his telescope to the sky, little did he know that he would prove the heliocentric model of the solar system, and disprove the geocentric model.  One of his key discoveries was the phases of Venus.  The phases helped show that Venus went around the Sun and not around the Earth as had been previously believed.

The last thing that is strange about Venus, and may be the strangest of them all, is how the Sun transverses the Venutian sky.  One most planets, the Sun rises in the east and sets in the west.  Sometimes, it can go retrograde (but only on Mercury), but on Venus, the Sun only goes from west to east in the sky, rising in the west and setting in the west.