APOD 4.5 (Scintillating)

I picked this one because it shows how little difference there is between a planet and a star.  The planet Mars and the star Regulus both make the same apparent "path" through the sky when their photo is taken by a swinging camera, but since Mars' light scatters less before it arrives here and Regulus' light is less consistent, it makes Mars' path have a constant red color while Regulus' path appears to be rainbow-colored.

Astrocast #212 (GPS navigation)

Worth two observation hours per Mr. Percival

On this episode of astrocast, we learn about GPS satellites, ranging from their invention to the way they function.

As people might expect, GPS was originally a secret military technology (like many great household inventions before it).  The government needed its soldiers to be able to complete missions without having to consult maps (which might ruin a mission due to a lack of landmarks, a false sense of direction, an old map, the soldiers being spotted by the light they are using to see the map, or a host of other possibilities).  So, they used the accuracy of the newly-invented atomic clock to make satellites that, when at least three were in range of you, could tell you your exact position at any given time.  However, when the GPS finally became public, the military was apparently afraid that terrorists could use GPS to guide bombs to a specific location from a long distance, so they programmed in random timing errors, decreasing accuracy significantly.

The errors are gone now, and with many modern devices using GPS along with other technologies (cell phone towers, wireless networks, etc.), GPS is accurate to within the possible timing errors, at the best of times resulting in a possible area less than the size of a Starbucks!

APOD 4.4 (Rio Morning Moonset)

Here, we see how quickly the moon moves across the sky, how quickly the light reflection off of it changes as its light passes through the low-hanging dust clouds in the atmosphere, and how quickly the city of Rio springs back to life each morning (since all the birds in the photo were only captured in the last frame).  When the moon is almost directly overhead, its reflected light doesn't pass through much dust between the Moon and your eyes, giving you the full spectrum.  However, as the moon approaches the horizon, it passes through a lot of dust before it reaches you.  The dust absorbs the darker colors, only letting a small amount of the redder light through.  By the time the moon actually sets, there is so much dust blocking the light that the moon becomes almost invisible.

APOD 4.3 (Otherworldly Planet Rise)

I chose this one because it might become as familiar a sight to people of the future as sunrises are to us today: your day starting when a small little dot of intense light rises up in the sky and illuminates the world more than any other surrounding visible star.  This amazing photo was made on Earth, taking an early morning picture of Venus underneath a natural rock formation.

APOD 4.2 (The Milky Way Over Tenerife)

In this picture, you get a clear panoramic view of all the circumpolar constellations, as well as many of the spring constellations.  To give you an idea of just how dim the stars and galactic features actually are, that bright light illuminating the entire photo is the moon, which has been significantly overexposed in order to capture the dim light from the surrounding constellations.  Also, the band that covers the sky almost like a purple rainbow is half of our view of the galaxy we are currently traveling through: the Milky Way.

APOD 4.1 (It's Raining on Titan)

I chose this picture because Titan is the only object other than Earth that has shown evidence of stable bodies of surface liquids.  The only difference is that, while Earth's stable liquid is water, Titan's temperatures around -290 degrees Celsius mean that the processes of evaporation,, cloud formation, and rain are applied to liquid methane.  While this might mean we would have to look elsewhere for a second Earth, it does prove that Earth's water cycle could happen elsewhere naturally, making our hopes of finding a home after Earth much more likely.