APOD 4.7 (A Beautiful Trifid)

Since we are getting to the end of this year's APODs (not going to miss dealing with this interface), I thought we could wrap it all up by talking about one of the first constellations we learned in the newest APOD.  M20, or the Trifid Nebula, is a celestial object inside the constellation Sagitarius.  It actually consists of three different types of nebulae: red emission nebulae dominated by light emitted by hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark dust nebulae where dense dust clouds appear in silhouette.  The Trifid has a central red emission nebulae divided into three parts by a few dark dust nebulae, and surrounded by the blue haze of a blue reflection nebulae.

Astrocast #220 (Mass Extinction Events)

Worth two observation hours, per Mr. Percival

In this podcast, Fraser and Pamela talk about the different times in the past where the majority of the life on Earth died, how they died, and how we finally found out about it.  The most recent one, the asteroid that hit the Yucan peninsula, killed most of its victims either from the initial impact, the shock wave directly resulting from the initial impact, or the years of skies darkened by dust thrown up into low Earth orbit (where anything that had been living after the impact and shock wave that had been carried up there died) by the shock wave from the initial impact.  This caused many plants to die, causing many herbivores to starve, resulting in first-level carnivores to slowly die off.  This wave of one trophic level mostly dying off causing another to die off quickly found its way to the highest levels, where it had the most impact because the animals on top have the biggest appetites, and the fewest resources.

In the KT event, the most famous one, where over 70% of the land population and over 95% of the aquatic population died, it is believed that it was the result of a meteor that hit the Earth so hard at just the right angle that the shock wave built up on the opposite side of the world and either created a volcano of its own or pushed the volcanoes already there into overdrive, effectively kicking the populations back down as soon as they tried to rise up out of the rubble.

For the prior events, it gets exponentially harder to research the events because the Earth recycles its crust, so the older impact craters or volcanic debris either has already been recycled or is on the verge of being recycled.  This means that we mostly have to rely on how many died following each event (because it is more likely for fossils to be available from that time period somewhere in the world than it is for one specific point to still be analyzable).

APOD 4.6 (Farther Along)

I chose this image because it literally shows how far we have come in our space age technology.  It shows that some of our oldest probes (Voyager 1, Pioneer 10, Voyager 2, Pioneer 11) are far out there, Voyager 1 farthest out at a distance of 17.5 billion kilometers, or 16 light-hours from the Sun (by comparison, the Earth is 8 light minutes away from the sun), Pioneer 10 close behind at 15.4 billion kilometers, although it is on the opposite side of the solar system from the four other satellites highlighted.  The New Horizons probe, launched a few years ago, should reach Pluto in 2015, which should give us much more data on the dwarf planet when it arrives.

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.