Surpise! (To me, apparently) Mercury is crossing the Sun!

This post will take the following form:
I) Safety Notes
II) Mercury crossing the Sun
III) Some thoughts on How I Feel About This (feel free to skip)
IV Final Notes

I)
.....First, before I get into anything else, NEVER EVER EVER LOOK DIRECTLY AT THE SUN!  Okay, fine, if one is looking at the Sun at sunrise or sunset, it is much safer for reasons that I'll go into, but I don't want that to weaken the gist of LOOKING AT THE SUN IS DANGEROUS, ESPECIALLY IF ONE WERE SELF-DESTRUCTIVE ENOUGH TO USE BINOCULARS OR A TELESCOPE (without a filter).

.....Before going on, let's go through why (I am never going to say "do / don't do a thing without explaining why), and it has to do with quantum mechanics.

.....No! Don't leave! Please!

....."Quantum" means, "one, one piece" and in this case it means "one interaction at a time." Light is a way (the way, actually) of transmitting energy through vacuum, and quantum mechanics has us treat light as a single piece of energy. (Let's call this piece a "photon".) Energy, of course, can do things, and enough energy in the wrong place can do things like kicking the electrons of an atom around, or breaking apart molecules, or even breaking apart atoms. However, light can't "team up".  If a certain amount of energy is required to break a link between atoms, then a whole bunch of lower-energy photons will do nothing to it, and will in fact be entirely ignored. If something is going to happen, then it has to happen all at once, with all of the energy contained in one photon.  This is why high-energy photons (ultraviolet light, gamma rays, X-rays) can be dangerous and are to be avoided, while low-energy photons (red light, infrared light, radio waves) have so small an effect that one an use a red light to read in the dark, without the light breaking up the molecules your eye constantly makes allowing for low-light vision.

.....Also, the hotter an object is, the more energetic the types of light the object is giving off in number.  Your stove is never going to be a dangerous gamma rays source.  If you Hulk out while cooking, I'm afraid that's on you.  The Sun however, being around 10,000 F, is giving off a lot of UV light, so staring at the Sun is giving a chance for these little jerks to get into your eyes and kick things around.

Dangerous and harmful. Also shown: unjustifiably risky action

.....Does this mean that you can't see the shadow of Mercury moving across the Sun? You sill have options!  If you have specially made eclipse glasses (something made explicitly to protect against UV light), you might be able to see the dot of Mercury across the Sun.  (If you can't do NOT use binoculars on the outside of the filters!)

....But there is an easy way!  If you have a pair of binoculars (or a telescope), then (NOT LOOKING IN THE ACTUAL TELESCOPE OR BINOCULARS) hold the binoculars so that the image falls on a piece of paper or cardboard.  In this way, none of the UV light will enter your helpless and currently irreplaceable eyes.

II)

.....Mercury orbits more closely to the Sun than the Earth does, cycling in 88 days as opposed to our 365-day trip, so that means that each 116 days (Do you want to see the math? 'Cause I'll show you.) Mercury passes the Earth in their orbits.  Even with that, it is a very rare event for the disk of Mercury to pass in front of the Sun This is because Mercury's orbit is tilted by seven degrees to the Earth's orbit, and the Sun is only half a degree across against the sky, so most time Mercury passes above or below the Sun.

.....The picture below shows the path of the transit, though sadly in Eastern Standard Time (subtract one hour, of course, for Central Time).  Mercury will take several hours to move across the Sun, so hopefully eberyone will get a chance to see it!  I know that I hope to have my telescope out until I have to go to work at about 10:00 AM.

image from NASA

III)
.....I am embarrassed by being caught by surprise like this. Observational astronomy has always been one of the most enjoyable aspects of my life, and getting taken by surprise like this kinda illustrates how my horizons have collapsed into day-to-day concerns as I don't have the professional goals that I once had.  I promise to try and stay ahead of these things to keep the good connections between me and past-me.

IV)
RE-READ SECTION ONE!

Why does the Moon turn red?

.....I'm going to try to start from as close to basics as I can get, so if I start at an a level that's obvious to you, please give me a moment to catch up to you.  Let's start with the most basic building blocks.

.....Light is energy moving from one place to another.  Anything that has a temperature above absolute zero (anything that exists) will give off energy, and therefore will give off light.  (Human beings have a body temperature of 98.6 degrees Fahrenheit which is 310 K in the Kelvin scale, and human beings do give off light.)

.....All light is not equal, it exists on a spectrum in which the amount of energy carried by a given "piece" of light energy* defines where it falls.  The visible spectrum of light is just a part of this, but the principles that the define the whole spectrum are shown here.  In addition to pretending that light is a particle, we can also pretend that light is a wave*.  (Yes, both notes reference the same end note.)  The more energetic the wave, the shorter the wavelength.  An object emitting energy emits energy along the entire spectrum, but what type of light  is mostly emitted depends on the temperature of the object.  NOW I'm getting to the point where this becomes germane.

.....One last bit of backstory: the Sun has a temperature of 6000 K (around ten thousand Fahrenheit), and emits most of its light in (surprise not surprise) the visible spectrum, and that combination of every color of light results in the Sun emitting white light. (At 310 K, the human body emits light in the infrared range.)



.....We can see through air, but that does not mean that nothing is there; it is made up mostly nitrogen and oxygen molecules.  As these waves pass among the atoms and molecules in air, the waves are scattered, and the shorter the wavelength, the MORE the light is scattered.

.....When that white light hits the Earth's atmosphere, the blue light, with the shortest wavelength gets scattered first, and it's scattered a LOT.  Across the entire sky, as a matter of fact.

.....The more air the light passes through, the more light is scattered.  Violet, indigo, and blue very quickly, and then as the Sun gets lower in the sky green light is removed, then yellow, leaving only orange and red.  When the Sun is lower still, only the red is left.  The red light is scattered least, but still scattered, and in fact the red light is scattered so little that it leaks into the shadow of the Earth, so that what would be blackness in the shadow of an airless planet becomes a cone of red.

*This is not physically "real", but it is a very useful and helpful way of looking at the universe despite not being, for the lack of  a better word, "true".

(As it turns out, the Moon will also be on the nearer part of its orbit to the Earth, meaning it will

The Sky

There are a lot of different entry points to learning astronomy.  Some people become interested because of science fiction, or the incredible images produced by space-based telescopes, or by an interest in the physics involved ... or, from standing outside at night looking up at the sky.  If you are brought to this interest by the last path, this might well be your starting point.

(As this is one of the possible starting points, some of the language in this section may assume that you are just beginning your trip through these astronomy pages.  If you are not starting here, and have been redirected here from somewhere else so that this language seems clumsy to you, at least know that there was some active thought that went into this.)

The study of the universe will involve shifting our point of view across tremendous distances, from the scale of atoms to that of clusters of galaxies.  One starting point is that of our ancestors thousands of years ago, standing on the ground looking up at the sky.  While this sounds simple, a way of describing what we see in the sky can be difficult.  We are looking at thousands of objects on what seems like the inside of a bowl, extended to a sphere, on which the position of an individual object depends on where on Earth we are, and the time we are looking at it.

This presents a challenge that is commonly answered using an armillary sphere such as the one below.

Building examples is beyond the scope of this work.  Even two-dimensional representations of three dimensional objects (especially those you will want to envision well enough to rotate in your mind) will be challenging, some we will work up to something like the thing below:

                If you don’t have experience working with the celestial sphere, it will be helpful to work through the steps by which it is constructed.  After all, our final display contains a lot of information.  Let us start by considering the stars visible on a clear, moonless night. There certainly are a lot of them, there being about six thousand stars visible to the eye spread across the entirety of the sky.  

Now: imagine yourself looking at stars.

Clearly this diagram is wrong.  

That image almost certainly doesn’t look like you, and there are millions of people who could be reading this for whom the diagram, crude as it is, still manages to be different in major ways from themselves.  Before we move on to the obvious stuff that I’ve missed, I’d like to take a moment to defend this weak version of you, and to explain why I’m going to rely on diagrams even at some times when I could produce a photograph. [Link to come]

Even more importantly, the Earth isn’t there.  What does the Earth around you look like?  As far as this goes, we really don’t care.  This is another instance in which an insistence on being exact would only make the situation far more complicated and confused than it really needed to be.  Drawing in hills and holes and houses would camouflage where the similarities between different areas are far more teachable than getting the shape of the neighbor’s roof right. 

Where does the effect of the Earth come in?  Consider the following diagram of a person standing on the Earth:

 Again, that diagram is extremely wrong.  Clearly, the person is actually much, MUCH larger than a person, but now have another benefit of using a sketch instead of a photograph.  We can start from a point that is obviously wrong, and then start moving toward a more correct view.  Doing this, we can look for patterns that appear as circumstances change and extend those patterns to mimic the real world in a useful way.  Looking at our starting image, we see that the mini-Earth blocks some of the stars.  Let’s make the mini-Earth larger and see what changes.

The larger mini-Earth blocks more stars.  If we look at these two diagrams, and perhaps keep making drawings with the Earth as a larger and larger circle, we can predict that by the time that the sketch of the Earth reaches the size of the Earth, then the lines showing the part of the sky that is blocked is a single straight line.  Thus, I can now draw the visible sky as it is drawn below, with half of stars visible, and half of the stars blocked.

Now we are able to add the stars.  When we stand in a room with other people, a step in any direction changes your view of where people are in relation to each other.  If you are at the center of a larger room, taking one step results in a lesser effect. All these stars are so far away from the Earth to be essentially infinitely far away, and therefore we can treat them as basically fixed to the sphere of the sky.  

At this moment, we stand inside the celestial sphere with half of the sky blocked. We are not done, because the Earth rotates.  The Earth spins around an axis, but it carries us, the air around us, any little things that we toss up into the air at the same rate.  This motion is so universal to our experience that when we look out at the rest of the universe, it appears that we are standing still and the universe is moving around us.  (This illusion took a long time to overcome in our understanding of the universe [Link to come].)

If we were standing at one of the poles (I will use the North Pole just because most people live in the Northern Hemisphere), then we would feel ourselves to be at the top of the spinning Earth, and the stars in the sky would be spinning about us.  A star directly about us (and there is one very close to that [Link To Come] - at least at currently [Link To Come]) would stay in that place, and every other star would trace a circle around the sky, always staying at the same altitude [Link to Come] above the horizon.  Should we take one step away from the pole (which is honestly not a place humans are well-adapted to live), this view would change.

Were we to walk to the equator, we would see one pole on one horizon and the other pole on the opposite horizon.  The stars would no longer be tracing constant circles in the sky, but each star would be above the horizon for half of the day, rising in the east and setting in the west.  If we imagine walking from the equator to the pole, our view of the rotating sky would change from the second situation to the first.



We are able to get something positive out of the illusion of the sky’s motion.  The point in the sky directly above one of the poles, the point which would appear to not move if we were standing at a pole looking straight up, is the point on this imaginary sphere straight out from the Earth’s axis.  We can define these points as the North and South Celestial Poles.  On the celestial sphere, this provides one of the axes on which we can define an object’s position on the celestial sphere.  On the diagram, we can note these points, and define the Celestial Equator as the great circle exactly hallway between the poles.  We can define the position of a celestial object by the angle made by a line from the center to the object and the plane of the Celestial Equator, and that's handled here [Link to come] for convenience's sake.
  

Were we to move to a latitude of 10˚ north of the equator, the point on the sky where the Earth’s axis passing through the Earth’s North Pole strikes the sky (The North Celestial Pole, or NCP) will be 10˚ above the horizon, and this will keep tracking until we reach the pole at a latitude of 90˚ north, and 90˚ above the equator.  This means that from any latitude (I’ll use the Greek letter lambda – [Argh! How do I do this in Blogger?] – to denote this angle in diagrams) the North Celestial Pole will be that angle above the horizon.

Stars close to the pole in the sky will always be visible above the horizon.  Often, a circle will be added in to denote this mark.  Any star inside this circle will always be above the horizon, and can be observed any time, any clear night. We have to pay for this; there will be an equally sized area around the other pole that will be invisible to us at every point during the year.

The next complication is that the Earth's rotation is tilted by 23.5˚ to the Earth's orbit. 
This means that the path of the Sun in the sky (over the day and over the year) is tilted to the Earth’s orbit, and that will carry the Moon and the planets with it.  That accounts for the remaining set of circles, but that topic has enough interest to deserve its own page. [Link to come]