Hey folks, I’m back from a week rambling around Downeast Maine, so while the frequency of posts has suffered, the material is overflowing!  More to come on that later, but for now, it’s time for a weekly snapshot of astronomical items of interest.

This week, it’s the Perseid meteor shower – peaking (unfortunately) during daylight hours for North America, the Perseids typically put on a good show with bright, frequent meteors.  Start looking for them late Friday, and through Saturday night.  Unfortunately, the waning gibbous moon will be out, and the skies won’t be particularly dark, but the brighter Perseids ought to be able to cut through the shine.

The apparent point of origin is (as I’ve described before), essentially the direction that Earth is travelling at the moment, with some geometrical adjustments for the stream of debris that crosses our orbit and causes the shower – in this case, that debris comes from the dust-shedding of Comet Swift-Tuttle (109P).  These meteors will appear to radiate from the constellation Perseus – hence the name.

As with most good meteor showers, you don’t really have to focus on this part of the sky to see them, but knowing when that point is above the horizon is a good start.  Look to the northeast late Friday night, just east (below) the “W” of Cassiopeia, and you should see a show – possibly as many as one per minute.

Persieds

Perseid origin – Midnight on 8/11.  via Stellarium.

The other item of interest this week is the partial lunar eclipse that we here in North America will not be able to see at all (but eastern Europe, Africa, Asia and Australia will catch it).  The interesting part about this is that it is not at ALL a coincidence that we’re seeing a lunar eclipse of the full Moon just 2 weeks before a total solar eclipse at new Moon.  In fact, it sort of begs the question as to why we don’t see an eclipse of some sort every month?

Again, the answer lies in geometry – the Moon’s orbit around Earth is tilted about 5 degrees with respect to the ecliptic (again, that imaginary plane of the solar system’s planetary disk).  So for roughly half the year, the Moon is too “high”, above the ecliptic plane and crosses above the Sun from our perspective, and for the other half, it’s too low and crosses below the sun.  The same is true for lunar eclipses at full Moons, but because the Earth is so much bigger than the Moon (and close to it), lunar eclipses have a bit broader window.  There are generally two times of the year when the alignment is, as Goldilocks would say, “Just Right”, and then it’s a matter of timing – but when this happens, the Moon is lined up with the Earth on BOTH sides (a lineup described with a wonderful term called syzygy) and so we get both eclipses in the same month!  It’s also no coincidence that we’ve named the planetary disk the ecliptic – whenever the full or new Moon approaches it, eclipses are possible.  Creative, I know.  I’ve attempted to diagram this relationship here.

Lunar Geometry

The Moon’s high and low points precess a bit – even though I’ve drawn the Moon’s orbit as if it is always aligned with some point out in space, if you were to draw a line from the low point of its orbit to the high, you’d notice this line is also slowly revolving such that the period between eclipse seasons is not exactly 2 per calendar year, but 2 plus a little bit.  So on rare occasions, we have three “eclipse seasons” in a single calendar year.  At any one spot on the planet, though, a total eclipse is a very rare thing.

Only 2 weeks before our chance to see one!

Get Out There!
Troy
flying-squirrel.org

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