Tides

The tides on Earth are driven by gravity, but not Earth's gravity.  The main reason we have tides is from the Moon.  The Sun affects the tides to some extent, but the Moon is the chief driver of the tides.  The gravitational pull from the Moon on the Earth attracts the oceans in a way to create tides.

High tides occur when the Moon is directly overhead or overhead on the opposite side of the Earth. Low tides are when the Moon is on the horizon.  When the Moon is directly overhead, the Moon is pulling water towards it, creating a bulge.  This is called a sublunar tide.  When the Moon is at the other side, it is pulling the Earth away from the water.  We call this an antipodal tide.  At the horizon, it is basically pulling the water along the surface of the Earth, and we have low tide.

In reality, since the Earth is rotating, the tides actually follow a couple hours after the location of the Moon.  But for our purposes, we can safely assume the Moon is directly overhead or at the horizon.

During the lunar cycle, there are times when the Sun, Earth, and Moon line up in a condition called syzygy.  These are when the Moon is full or at new phase.  Tides are higher than normal because of the combined gravitational attraction of the Sun and the Moon.  During the Full Moon and New Moon, the high tides are called spring tides. During the first quarter or third quarter, the Moon and the Sun are 90° apart in the sky and high tides are at the lowest heights.  We refer to these high tides as neap tides.

If there hadn't been a Moon, we would still have tides, but they would only be affected by the Sun.  High tides and low tides would be much different than we have today.  In fact, if there hadn't been a Moon, ground-based life might not exist.  Biologists believe that life began in the oceans and as tides rose and fell, some of that life might have been left behind on the shores, especially during spring tides.  This would force that life to adapt to life on land and evolve into air-breathing creatures.  If there hadn't been a Moon, intelligent life might still have evolved, but would have developed in cetaceans, rather than primates.

The Moon also affects atmospheric tides, but since air is less dense than water, the tides are not as pronounced.  Atmospheric tides do add to weather and climate on Earth.

Lastly, as mentioned in a previous post (The Origin of the Moon), the Moon is slowly receding from the Earth.  As it gets farther and farther away, the size of the tides will decrease.  Since the recession is only 2 cm/century, it will take millenia for the size of the tides to be noticable.

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.

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