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.

Perseid Meteor Shower

If you go outside at night this week, you might be able to see an annual event called the Perseid meteor shower.  It occurs yearly when the Earth passes through the comet Swift-Tuttle's debris trail.  This year, the shower will peak on the night of August 12-13, with the maximum being before dawn on August 13th.

The Perseids are called that because if you were to trace the origin of the meteors, they would appear to originate in the constellation Perseus.   This origination point is called the radiant, since the meteors seem to radiate out from this point.

When we experience meteor showers, the Earth passes through a debris trail left by comets or asteroids that have orbits that cross near our orbit around the Sun.  The debris is caught by the Earth and falls through the sky and what we see is the meteor.

There are a couple of definitions you should learn.

• Meteoroid - the actual object falling through the sky.  This can be as tiny as a dust particle or as big as an asteroid (which we really don't want)
• Meteor - this is actually what we see.  As the meteoroid falls through the atmosphere, friction causes the gas to heat up around the meteoroid and glows.
• Meteorite - if the meteoroid is large enough, it will not completely burn up as it falls and what is left when it hits the ground is a meteorite.

For your viewing pleasure, this week, Perseus is a little north of the Moon.

Supermoon

I probably should have posted this a couple of days ago, but oh well.

The Supermoon of 2014 just occured this past weekend (August 10). 

The Supermoon is not the newest superhero of DC or Marvel.

The Supermoon is the full moon happened to occur during perigee.  The Moon appeared the largest because it happened to be at its closest in its orbit to Earth.  Tides (which will be discussed further in a future post) happened to be a tad higher, but nothing else really is affected by a Supermoon.  It's just cool to see the Moon so large.

Return to the Moon

We have not been to the Moon since Apollo 17 in 1972.  The reason the United States went to the Moon, originally, was not for science or exploration, but rather for political reasons.  The US wanted to beat the Soviet Union to the Moon.  On July 20, 1969, when Neil Armstrong made one small step for man and a giant leap for all mankind, a human being stood on another celestial body other than Earth for the first time in history.

Why haven't we been back?  One, it is extremely expensive to travel to the Moon.  Not only do we have to have enough fuel to get there, but we need enough to get back.  We'd also have to worry about keeping the astronauts safe while on the Moon.  Two, there really is no economic or political gain from going to the Moon.  At the moment, the only gains we would receive would be purely scientific.  There is no profit to travelling to the Moon, though sometime in the future, it may be profitable to mine the Moon.  Politically, it wouldn't make one country better than any other.  The only advantage would be if there was a multi-nation coalition to go the Moon and make it worthwhile for all humanity.  Lastly, we don't have the technology to go back to the Moon.  A whole class of new spacecraft would have to be designed, tested, and constructed for man to go to the Moon once again.

Why is this important?  There has been talk of a crewed mission to Mars, which is all well and good.  But to skip going back to the Moon first would be a huge mistake.  The Moon is much easier to get to from the Earth than Mars; it would only take a few days travel to get to the Moon, with a round trip only taking about a week.  To get to Mars, it would require at least 6 months of travel from Earth to Mars and almost two years for a round trip.  If humanity built a lunar base, it would be easier to use as a launching point for exploration of the rest of the Solar System.  It would require less energy to launch a ship from the Moon than the Earth because the Moon is much smaller and its gravity would not work as hard against launching a rocket or spacecraft.  Once we set up a permanent presence on the lunar surface, exploration of the Solar System should follow, with Mars being the most logical first step.

Another nice thing about using the Moon as a launching pad is that the materials needed to build rockets and habitats and create fuel for spacecraft are already on the Moon.  The challenge would be to harvest the material and convert it into useful products. That is obviously many years in the future, but we still need to return to the Moon before thinking about going to Mars.

The Origin of the Moon

When we first explored the Moon, we weren't sure what we would find.  We expected to find similar material that we find on Earth.  What we found is even more unusual.

First, why did we expect similar material on the Moon?  Based on the location of the Earth in the Solar System (and by default, the location of the Moon), we expected to find refractory elements because they have a higher boiling point than volatile elements, i.e. they vaporize at high temperatures which in the early Solar System included the location of the Earth.  All planets close to the Sun are generally made up of this type of material because they are close to the Sun.  This is why we believed that terrestrial planets are close to their central star and Jovian planets are far from their star.  Because of this, we expected to find the Moon was made up of refractory elements.

However, when we actually went to the Moon, we discovered something really strange; the composition of the Moon is nearly identical to that of the interior of the Earth.  Not that we found the same material, but that the concentrations were the same.  So what does this tell us?

This tells us that when the Earth was very young, something collided with the infant Earth to create the Moon.  A Mars-sized body collided with the Earth to create the Moon and to leave some material on the Earth.  This helps explain three things:

1. The Moon's composition and why it is nearly identical to the Earth's composition
2. The Moon is receding from Earth. Recall that the Moon is moving about 2 cm away from the Earth every century (See blog post on the eclipses)
3. The 1:1 Moon-Earth resonance since the Moon formed from the Earth

Daily Galaxy

Synodic Period vs. Sidereal Period

As mentioned before, the sidereal period of an object depends on that object's alignment with the body it orbits and with a distant, background star.  When we talk about the Moon, we need to clarify the difference between the sidereal month and the synodic month.

Last time, we talked about the lunar resonance, explaining why the same face of the Moon is always pointed towards Earth.  When we measure the time from Full Moon to Full Moon (or New Moon to New Moon), it takes about 29.5 days to complete one cycle of phases.  We call this the synodic month and it is also the reason why months are approximately 30 days.  But the sidereal month is only 27.3 days.  The reason why it is shorter than the synodic month is because as the Moon orbits the Earth, the Earth is also orbiting the Sun.

Lunar Resonance

Despite what my wife thinks (and the Pink Floyd albums says), there is no such thing as a "Dark Side of the Moon". Granted, the face of the Moon facing away from the Sun is dark, but that face changes as the Moon orbits the Earth.  A better description of the faces of the Moon would be to call them the Near Side of the Moon and the Far Side of the Moon.  The near side is the face that always pointed to the Earth, and the far side is always pointed away.  Why is this?

The reason why we should use near side and far side is because the Moon is in a 1:1 resonance with the Earth.  Remember that the Sun-Mercury system has a 3:2 resonance, so for every two orbits around the Sun, Mercury rotates three times on its axis.  For the Moon-Earth system, this means that for every one complete orbit around the Earth, the Moon rotates on its axis just one time.  Because of this, the same side of the Moon is always facing Earth.  It wasn't until 1959, when the Soviet Union's Luna 3 space probe photographed the far side.  In 1968, it wasn't observed by human eyes for the first time during the Apollo 8 mission.