Star vs Planet

As a child, I learned that the way to distinguish a star from a planet in the night sky was to determine if it twinkles. That is the fundamental visual difference in these two types of celestial bodies. The reason behind this visual phenomenon has to do with the object's angular size, or its apparent size in our sky resulting from its actual size and distance. A star is a very large object but is at a tremendously large distance from us. By contrast, a planet is smaller, but very, very much closer to us. The result is a star being a point source of light while a planet appears as a disk (albeit a very small disk). Think of it as a star emitting a stream of photons in single file compared to a planet emitting a wide bundle of photons. Upon entering the Earth's turbulent atmosphere, the light from both the star and planet will be refracted (bent) to the same degree, but the skinny photon stream from the star will be affected more by the atmosphere than the thick photon stream from the planet. As the refractive index of the atmosphere varies from the turbulence in its different layers, the star will twinkle while the planet will shine with a steady light. It's like comparing a six-inch wide creek with a 100-foot wide river. Throw a rock in each and the creek will be affected to a larger degree than the river.

We can see this visual difference for a couple more weeks in the night sky. Look to the southwest just as the sky darkens enough to reveal the bright star Antares and the planet Mars. They are currently 5 or so degrees apart, with Antares being below Mars. The ruddy surface of Mars and the M-type spectral characteristic of Antares give them a very similar color, which is reddish-orange. As explained above, Antares twinkles because it has an apparent size in our sky of .0471 arc seconds (1.12 billion km in diameter at 550 light years away--virtually a point) compared to the apparent size of Mars of 6.1 arc seconds (6,800 km in diameter at 227,680,000 km away.)

The photograph was taken on September 29th, 2014, so the moon is in the scene also.

Optical effects

An interest in observational astronomy also leads, at least in my case, to an interest in the science of how the telescope reflects and refracts the light coming from distant regions of space with its mirrors and lenses to form the image that I see through the eyepiece. Then there's also the brain's role in interpreting the image that the eye sends to it. Here are a couple of more down to Earth effects that I witnessed (or created with the last image) that are also examples of optical phenomena.

First is this that I noticed on the wall of my bedroom one morning. I couldn't tell exactly what it was at first until I imagined the image flipped upside down.

Seeing it this way makes it more obvious. It's an extended cab pickup truck that was parked across the street, and its image was formed on my bedroom wall because of a very tiny gap in the dark drapes that cover the window. The gap was acting like a pinhole camera and projecting the image across the room and onto the wall. Since the pickup was white and sitting in full sunlight, it's the brightest and therefore most obvious thing in the image.

Here's something else that gets your brain involved. An ordinary camera can take 3D images with a simple trick. First, take a photo. Then, depending on how far away your subject is, take a step a few inches (for close-up subjects) or a couple of feet (for distant subjects) to the left and take another photo, identical to the first except for the slightly different point of view. Put both pictures side by side in front of you so that you can converge your eyes (in other words, going slightly cross-eyed) so that the two pictures merge into one. If you can get away with this without getting a headache, your brain will interpret the image in 3D. Click on the image above to enlarge it for an easier view. This photo is at Prague Castle in the city of Prague, Czech Republic. I've taken a few pictures like this at various places, and the results are usually quite striking.

I spent some time outside and I saw a few things of interest. First was a bright spot in the sky that I saw well before the sun set. I spied it through my telescope and saw it to be a weather balloon. My optical set-up was giving me only 16x magnification, so the photograph I took didn't show much detail, so I enhanced it (with debatable success which you see at left) to show the balloon's components. The radiosonde (weather instrument package) that's hanging from the balloon is the small spot at about the 5 o'clock position. Click for a larger image.

Second was Jupiter. I had heard that starting a few months ago, this great planet's southern equatorial belt had faded to almost invisibility. For my entire life, I've always seen Jupiter with a prominent belt in both its northern and southern hemispheres, but when I turned my telescope to it tonight, I saw that indeed the southern belt was gone. This isn't unheard of. Jupiter has lost a belt several times over the last 100 years. I just hadn't seen it in this state before. It's estimated that the belt will reappear sometime in the next few months. As a bonus, Io, Jupiter's innermost large moon, was just off the western limb of the planet. Two and a half hours later, I saw that it had moved the full planet's diameter away. Planetary dynamics at work!

Next was the International Space Station (ISS). If you ever want to know when this (or any other bright satellite) will be making a pass in your sky, go to http://heavens-above.com. You can save your location by becoming a registered user, or select your city from a world-wide database.

This series of three photographs document the ISS's two minute pass over Tucson. Each photo was shot at 400 ASA, but using an exposure time of 8 seconds. Therefore, the ISS appears as a streak in contrast to the stars as it moved. This pass was a full hour after sunset, so the space station disappeared into the Earth's shadow by the time it was overhead. (A pass occurring closer to sunset can be seen nearly horizon to horizon.) It showed an obvious reddening as it dimmed from sight and the astronauts onboard experienced their sunset.

Here it is a little closer. It has a speed of 17,000 miles per hour, so during the 8 second exposure, it covered a distance of 38 miles. Being a junkie for numbers, I also figured out the station's apparent size in our sky. Being 250 miles up, with the longest dimension being 356 feet, the fraction of a complete arc the station subtends is 55 arc seconds. The equation is 55"=(.06742/(250 x 2 x Pi)) x 1,296,000

The values used are:

.06742=length of space station in miles

250=orbital height in miles

1,296,000=number of arc seconds in a complete circle

For example, this compares with Jupiter's maximum size of 49", so with a suitable tracking telescope, it's possible to see details in the space station's structure.

This last photo illustrates how it fell into darkness behind the Earth. I've labeled the background constellations (with two notable stars in parentheses). My camera was pointing almost straight up here.

Developing Storm

I was looking to the sky again today, like I frequently do, and noticed an opportunity to make a series of photos of storm clouds building over the Santa Catalina mountains north of Tucson. I set up my camera on a tripod and pointed it roughly north northeast toward an evolving cumulus congestus cloud. The photo at left is one of 54 shots that I made, each about 1 minute apart, and shows the clouds soon after I started. During the shoot, some closer, low-level clouds moved in to obscure the view, but not before I was able to capture some interesting dynamics of the storm.

Go to the website below to see the time lapse.

http://timelapsecreator.com/creator/view/72157624733437218/1000/800

Perseid report, August 13, 2010

The sky appeared to be a little more hazy tonight than last night, but it was still cloudless with a limiting magnitude of perhaps 4. I was outside at about 1:50 AM once again, and the Perseids presented its best meteors at the start of my observing. At 1:57 AM, a bright meteor of magnitude 0 shot through the heart of Perseus, leaving a glowing trail that persisted for 3-4 seconds. Three minutes later, a similar meteor laid a trail which glowed for 2-3 seconds. A brief gap ensued, but then I saw three moderately bright meteors within a span of two minutes. The first occurred at 2:09 AM and flared to a bright magnitude 1, followed by two of about magnitude 2.5. The rest during tonight's 30 minute observation are as follows:

Time/magnitude

2:12/3.0

2:16/1.5 -- this one was very short in length, covering not much more than a single degree as it must have been coming almost directly toward me.

2:17/3.0

2:21/2.0 -- I really noticed the speed of this one as it raced far from the radiant through Delphinus and Aquila in about 1/3 of a second. A typical satellite orbits the planet at a leisurely pace of about 7 kilometers per second. Not only are the Perseids traveling about 8 times faster, they are much closer to me. Both of those factors as well as the perspective of this particular one added up to an astounding visual speed.

Even though I kept my observations for the last two nights brief, I was treated to some nice meteors. I'll look forward to the Perseids again next year. Before that, though, there are the Leonids in November and the Geminids in December! But for now, it's off to bed.

Perseid report, Aug 12, 2010

The night shift that I keep at work is perfect for early morning stargazing, and there's no better time to view a meteor shower than during the small hours. It just depends on how active the shower decides to be. So last night I arrived home from work, changed clothes, cracked open a beer, and parked myself under the unusually clear skies of August 12th to see how many Perseids I could witness.

I wasn't that impressed.

Granted, I was viewing from the middle of Tucson where city lights mask the faint meteors, but I thought I'd see a lot more. I noted the time and estimated the brightness of each meteor I saw, and here are the results--

Time/Magnitude

1:50/2.0

1:53/2.5

1:59/2.5

2:01/1.5

2:07/3.5 (this meteor could not be traced to the radiant, so it wasn't a Perseid.)

2:09/1.5

2:11/2.5 (another non-Perseid going south through Taurus)

Up to this time the shower was really cooking. I was mainly concentrating on the area near the radiant and was certain that I was missing some meteors in other parts of the sky. Twice while waiting for one, I decided to examine some constellations toward the west and south and just happened to catch a meteor that would otherwise have been beyond my peripheral vision. But from this point onward, the meteors seemed to be more sporadic.

2:27/1.5

2:32/2.5

2:34/3.0

2:46/3.0

2:59/2.5

At this time I had had my fill and went back inside. While I didn't see a large number of meteors, my entire time outside was fulfilling since I also occupied myself with identifying some of the less prominent constellations to which I don't pay much attention: Cepheus, Triangulum, Delphinus, Cetus. . .I tried but couldn't see Equuleus. Those stars are just way too dim! The variable star Mira (in Cetus) was invisible, so it must be near minimum brightness. I'll watch it over the next few months to see if it makes its reappearance. And as always, the beautiful Pleiades star cluster twinkled at me in the east the entire time.

I'll do some more viewing tonight (the 13th), to see if the show is any better.

Arrival of the Perseids

The title may give the impression of a 1950's sci-fi alien invasion movie, but it's actually a meteor shower that occurs every year in the middle of August. A meteor, or "shooting star", is a bright streak of light in the night sky that results from a bit of space debris (a meteoroid) hitting the Earth's atmosphere and burning up from the kinetic energy of that collision. During the course of any clear night, you might be able to see one to a few random meteors. To qualify for a meteor shower, there needs to be many meteors that seem to originate from the same part of the sky with the same speed and occur at the same time each year. Their names come from the constellation from which the meteors appear to originate (such as Leonids, Orionids, Geminids, etc). However, the term "shower" doesn't mean that the sky will be lit up with meteors. Some of the weak annual showers only produce 5 to 10 meteors per hour. The Perseids are one of the more active ones, producing 50 to 80 meteors per hour at its peak during the nights of August 11th and 12th with a relative speed to the Earth of 50 kilometers per second.

The cause of the Perseids, like most meteor showers, is a comet. In this case, it is comet Swift-Tuttle, which was discovered on July 16, 1862, by Lewis Swift in Marathon, NY, and then independently by Horace Tuttle three days later at the Harvard College Observatory in Cambridge, MA. The 130-year orbit of this comet brings it just inside the orbit of the Earth at its closest point to the sun (perihelion), to well outside Pluto's orbit at its farthest (aphelion). Also, the tilt of its orbit is nearly perpendicular to the plane of the solar system. It's actually a little beyond perpendicular, thus giving it a retrograde orbit. When it visits Earth's neighborhood, it makes a looping path above the sun and across our northern sky. As it travels, it sheds bits of rock and dust which form a ring of debris along its entire orbit. When Earth passes through this debris, we get our meteor shower. Three years after this comet's discovery, it was the Italian astronomer Giovanni Schiaparelli who determined that Swift-Tuttle was the Perseids' progenitor by discovering that the orbit of the comet and the orbits of the meteoroids that cause the shower are the same. The radiant, or the spot in the sky where the meteors appear to originate, is in the northern constellation of Persus. When we look toward Perseus at this time of year, we are gazing down the path of Swift-Tuttle's orbit and will see the meteors diverging across the sky from this point. The above image simulates the area of the Perseid's radiant and the number of meteors over a 10 minute period.

It takes a full month, from July 23 to August 22, for the Earth to fully pass through the particle stream left behind by Swift-Tuttle. Since we are traveling about 30 kilometers per second around the sun, that means that the stream is about 72 million kilometers thick. At the very beginning or end of this period, we may expect to see 1 to 2 meteors per hour. But during the mornings of August 12 and 13th, the particle stream is at its thickets, and we'll see its peak rate of 50-80 per hour. Even then, the material in the stream is very sparse. The particles range form very small dust grains to pea-sized pebbles with about 150 kilometers separating each one.

Though you can certainly view the Perseids from anywhere, you'll do better by giving yourself a dark sky by getting as far away from city lights as possible. Fortunately, this year the moon will set shortly after sunset, so its glow will not hinder you from seeing the fainter meteors. Using your naked eye is best since the meteors will be traveling quickly and cover a large part of the sky. A telescope or binoculars would only be useful in observing the ionized vapor trails lingering behind by the brighter meteors. Furthermore, it's not necessary to know exactly where the radiant is. Just looking high in the northeast sky is sufficient. And even though the meteors will be visible at any time of night, you'll see more of them after midnight when you'll be riding the side of the Earth that'll be facing its direction of motion as well as allow full view of the radiant.

So, either take a moment to look up on August 12th, or park yourself in a lawn chair for some extended viewing of one of the year's best meteor showers.