What's in the Sky this Weekend?

If you get up early enough, in the pre-dawn eastern sky, you will be able to see Venus, Jupiter, and the crescent moon relatively close together in the sky.

Both Venus and Jupiter will be to the left of the Moon, with Venus being the brighter of the two objects.

Areavoices Via Astrobob

Also, there is Comet Jacques, a magnitude 5.66 comet in the constellation Cassiopeia, a W shaped (or M shaped) constellation in the northern sky.  It should be pretty easy to see with a good pair of binoculars.  Just look for the fuzzy object with a bluish-green hue.

Astronomy Picture of the Day

A Day on Venus

Venus has a unique day.  Granted, Mercury can have a day where the Sun travels from west to east in the sky, can even set again in the east only to rise again later, but this does not happen every day on Mercury.  Venus day is even weirder.

VENUS HAS A DAY LONGER THAN ITS YEAR



Yes, that is correct. It takes Venus less time to complete one revolution around the Sun than it takes to complete one full rotation on its axis.  Its orbital period is 224.7 Earth days.  Its rotational period is 243.0 Earth days.  Recall that both these are in reference to distant stars.

The orbital period, or sidereal period, of Venus is how long it takes for Venus, the Sun, and a distant star to be in the same configuration.  This is generally called its year.

The rotational period, or sidereal day, is how long it takes for a star to appear at the same longitude in the sky.  For Venus, this is 243 Earth days.  However, the solar day is how long it takes for the Sun to go from noon to noon.  For Venus, this is actually 117 Earth days.  How are these so different?

*This is my awesome artistic skills

 

As you can see, as Venus goes around the Sun, it's rotation is slow enough that it takes just over half a solar year for Venus to go from noon to noon.  However, it has just completed over half a rotation in that time.  Therefore, it takes almost another half a solar year to complete one full rotation on its axis.

 

There is another strange phenomena about the rotation of Venus.  On almost all the planets, the Sun generally rises in the east and sets in the west.  We've already discussed Mercury's strange day and Uranus day is strange as well, but Venus is the weirdest.  The Sun exclusively rises in the west and sets in the east.  We call this retrograde rotation.  If we view the solar system from above (i.e. looking down on the Earth's north pole), all the planets rotate counterclockwise (also called anticlockwise).  Venus, however, rotates clockwise.  

The way this is explained is by describing the inclination of Venus' orbit.  The Earth is tilted 23.5° with respect to its orbital axis.  Venus's inclination is 177°.  Why not 3°?  A 3° inclination would suggest that Venus rotates like all the other planets, counterclockwise (west to east).  By saying Venus has an inclination of 177°, we know that Venus rotates clockwise (east to west).  That is why the Sun rises in the west and sets in the east on Venus. 

 

 

Both the slow rotational speed of Venus and its almost 180° inclination are probably explained by the same thing: early in its creation, Venus was hit but a large planetoid body which caused it to flip upside down and considerably slowed down its rotation.

 

 

*Forgive my horrible drawings, as I am not an artist.  But I feel I should probably start using my own images instead of resorting to Google to find them.

 

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.

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.

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.





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.