Under Western Skies: ‘Shooting Stars’ and Eclipses

By Irwin Horowitz, 8-06-07

	Caption: (c) Dirk Obudzinski

Each year around August 12th, the Earth passes through a swarm of debris left in the wake of the passage of a comet known as Swift-Tuttle.  As these tiny particles streak through our upper atmosphere, they give rise to the annual Perseid meteor shower.  The name of the shower implies that the meteors appear to originate from the constellation of Perseus (this location is known as the radiant), located in the northeastern sky in the early morning hours at this time of year. 

Meteor showers are best viewed under dark skies when there is no moonlight to interfere, and this year the Perseids will occur around the time of New Moon.  In addition, they are better observed after local midnight, as referenced to the Sun. Since we observe daylight savings during the summer, this actually occurs after 1 a.m. local time for observers close to the central meridian of their time zones (and closer to 2 a.m. MDT for those of us in western Idaho which I suggested back in June should be in the Pacific time zone).  The reason for the popularity of this particular meteor shower for Northern hemisphere observers is that the weather is generally quite mild during this time of year and it is well placed in our sky.

The best way to view a meteor shower is not with binoculars or a telescope.  Rather, you are better off just laying a blanket on the ground or setting up a chaise lounge and lying on your back to look up at as much of the sky as you can see.  While they appear to come from the direction of Perseus, the streaks will be visible over the entire sky during the shower.

A total lunar eclipse will occur early on the morning of Tuesday, August 28th.  The entire event will be visible in the skies here in the western United States.  The partial phase of the eclipse begins shortly before 3 a.m. MDT (2 a.m. PDT) as the Moon first enters the darker umbral portion of the Earth’s shadow, while totality starts about an hour later at 3:52 a.m. MDT when it has been fully immersed.  The total phase of the eclipse concludes about 90 minutes later, around 5:22 a.m. MDT, while the partial eclipse will continue until 6:24 a.m. MDT.  At the start of the event, the Moon will be low in the southern part of the sky, and as the event unfolds, it will proceed towards the southwest, exiting the Earth’s shadow just before it sets below the horizon.  Those of you who hit the roads early to get to work should take a few moments to enjoy this spectacle that morning.

Total eclipses occur due to a nearly precise alignment between the Sun, Earth and Moon.  In the case of a lunar eclipse, the Earth is situated between the other two bodies.  It therefore can only occur at Full Moon.  Sunlight striking our world casts a shadow behind us in our orbit, which stretches for several hundred thousand miles.  When the Moon passes through this shadow, a lunar eclipse occurs.

Why don’t we observe lunar eclipses every month at Full Moon?  The answer to this question is because the orbit of the Moon around the Earth is inclined (tilted) by an angle of 5° relative to the orbit of the Earth around the Sun.  Most months, when the Moon is full, it will pass either above or below the shadow cast by the Earth through space.  However, usually about twice a year, the Moon will pass close enough to the shadow to produce either a partial or a total lunar eclipse.

During a total lunar eclipse, the Moon frequently takes on a reddish hue, due to the refraction of sunlight through the atmosphere of the Earth.  This effect is similar to what occurs when the Sun is low in the sky near sunrise and sunset and takes on a similar color.  It is due to the preferential scattering of shorter wavelength blue light by our atmosphere (this scattering also explains why a cloudless sky appears blue).  The longer wavelength red light tends to be transmitted through the atmosphere.  Sometimes when there is a lot of dust in our atmosphere, like after a major volcanic eruption, the totally eclipsed Moon takes on an even darker shade of red.

Most people who pay attention to such matters have had an opportunity to view a total lunar eclipse.  The reason for this is that such events are visible to anyone living in a part of the planet where it is nighttime during the event.  Since there are generally two lunar eclipses each year (though not always total or even partial), everyone has had plenty of opportunity to view at least one from their locale during their lifetime.

Not so regarding total solar eclipses.  For solar eclipses, the path of totality across the Earth’s surface is quite narrow (a few hundred miles wide at most).  People will travel all over the world to find themselves situated along that path to experience a few minutes in the Moon’s shadow.  I’ll discuss solar eclipses in greater detail in about 10 years (should this blog still be operating!), when we will have a front seat to our very own total solar eclipse (21 Aug 2017) here across the intermountain west (in Oregon, Idaho and Wyoming).
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This month’s skies continue to be dominated by the king of the planets, Jupiter.  It is the brightest object visible low in the south during these warm summer months.  The bright star beneath Jupiter is ruddy Antares, the rival of Mars.  It is located in the heart of Scorpius, the Scorpion and its deep red-orange color is due to the relatively low surface temperature of this star (less than 3000 K).  This star is a supergiant and is so large that if it were placed in the center of our own solar system, it would extend out beyond the orbit of Mars.  In a few billion years, our Sun will also expand outward, but not nearly as much as this overbloated behemoth.

For our early risers, start looking for the planet Mars in the east prior to sunrise.  It should be visible in Taurus, the Bull, south (to the right) of the Pleiades cluster at the start of the month, but passes north (to the left) of the orange star Aldebaran by month’s end.  It will appear a little brighter than the star.

The late summer is the ideal time for observing the heart of our galaxy, the Milky Way.  As I have previously discussed here, we live in a barred spiral galaxy, near the inner edge of a spiral arm about 2/3 of the way from the center towards the visible edge.  At this time of year, we are located on the side of the solar system closer to that center, and as a result, it is easily visible at night during our warm summer months.

The heart of the Milky Way is located in the constellation of Sagittarius (just east of Scorpius, it looks like a teapot).  When viewing the galaxy from a dark site at night, notice that the light appears to bulge outwards from the general plane of the disk.  This is the combined light from billions of stars, each so distant that they are too faint to be seen without a telescope.  A number of well known objects are found in this direction as well, especially several objects from the Messier catalog like M17, the Omega Nebula; M20, the Trifid Nebula; and M8, the Lagoon Nebula.

One last thing about the center of our galaxy:  Hidden by clouds of obscuring dust and gas is a monster lurking in the heart of our galactic home.  Astronomers have compelling evidence to suggest that a black hole, millions of times more massive than our Sun, resides at the center and everything in our galaxy orbits around it.  It is believed that every few hundred million years, a star wanders too close to this behemoth and is ripped apart by intense tidal forces and fed into the ravenous maw of the beast.  When this occurs, the core of our galaxy becomes incredibly luminous and emits tight beams with copious amounts of powerful radiation like gamma rays.  Events of this nature could even play a role in some historical mass extinctions here on Earth should we have had the misfortune of being in the path of this cosmic bull’s-eye.

This month’s astronomy events in the intermountain west begin with the annual meeting of the Astronomical League on 3-4 August in Portland, Oregon.  I have the pleasure of being one of the invited speakers at this event and am looking forward to a visit to the Rose City this weekend.

The following Friday, August 10th, the Boise Astronomical Society will hold their monthly membership meeting at the Discovery Center of Idaho on Myrtle Street in Boise.  The meeting begins at 7 p.m.  This month, Glen Knight will present a light-hearted discussion on how to prepare for a night of observing with a telescope.

On Saturday, August 11th, BAS members will be holding their own Perseid meteor watch at the remote Reynold’s Creek observing site in the Owyhee Mountains southwest of Boise.  Members and other interested parties are welcome to attend, but be aware that this is a primitive site with no facilities, water or electricity (except what you bring along with you).

The following weekend, from 16-19 August, is the annual Oregon Star Party.  This is one of the largest annual gatherings of amateur astronomers in the country.  It is located about 45 miles east of Prineville, Oregon, in the Ochoco National Forest.  If interested in attending, please check out their website for additional details.

As a peek into next month’s events, the annual Idaho Star Party will be held at Bruneau Dunes State Park over the weekend of 7-9 September.  I will provide more details about this event in next month’s column.

Until then, turn off your lights and turn on to astronomy!