Five of the solar system’s eight or nine planets (depending on how you count little Pluto) are visible among the stars in the night time sky. From closest to the sun out: Mercury, Venus, Mars, Jupiter and Saturn. Of these naked-eye planets, Mercury is the most difficult to see.

Mercury is tough to spot for two reasons. First, because it is the closest planet to the sun, and second, because it orbits the sun faster than all other planets.

Because Mercury is so close to the sun, it always appears near the sun in the sky from our earthly vantage point. The only times we can catch a glimpse of Mercury is when it is situated along its orbit some ways out to one side of the sun. And then we have to look either to the west, shortly after sunset, or to the east, shortly before sunrise, depending on which side of the sun Mercury is set off to.

Wednesday next week, Oct. 22, Mercury reaches a configuration astronomers call greatest elongation. At its greatest elongation, a planet appears as far from the sun as it ever gets. For us on earth, only two planets have greatest elongations: the two inner planets, Mercury and Venus.

The horizon at dawn. Jupiter is the yellow symbol, Mercury the greenish symbol. The map was created using John Walker's 'Your Sky.' Click the image to visit this website.

In fact, astronomer’s recognize two greatest elongations. Greatest eastern elongation is when a planet is as far to the east of the sun as it ever gets; greatest western elongation is when a planet is as far to the west of the sun as it ever gets. But don’t get caught looking the wrong way: to see a planet at it’s greatest western elongation, for example, look east shortly before sunrise.

Mercury is at greatest western elongation on Wednesday. Look east about a half hour before sunrise Wednesday, and for several days before and after, for a bright star low on the horizon. That will be Mercury.

But after next week, Mercury will quickly sink in the east back toward the sun, to again be lost in the sun’s glare.

Mercury is the fastest planet to orbit the sun, zipping around in just 88 days. So you have to look fast to catch a glimpse.

The moon and the planet Saturn can help you find Mercury next week. Saturn is about 20 degrees above the eastern horizon at dawn, and is easily the brightest object in the area. The moon next week is in its waning crescent phase, and will thin and sink to the east as the week wears on. Mercury will be low on the horizon below the moon and Saturn.

Mercury is the target for a space probe launched by NASA back in 2003. The space probe is called MErcury Surface, Space ENvironment, GEochemistry, and Ranging — or MESSENGER for short. MESSENGER is presently on a complex trajectory around the inner solar system bleeding speed to eventually orbit Mercury. In fact, two weeks ago, on Oct. 6, the little space probe zipped passed Mercury at an altitude of only 200 km, returning the most detailed pictures of a side of Mercury we can’t see from earth.

An image of Mercury taken recently by the Messenger space craft, courtesy the Messenger team at NASA. Click on the image to see a larger version.

You can see the pictures MESSENGER recently took and follow the mission’s progress at http://messenger.jhuapl.edu. MESSENGER is scheduled to blow past Mercury again on Sept. 29, 2009 and to arrive and go into orbit on March 18, 2011.

Pictures on the Internet are great, but reality is better. Take advantage of the late sunrises we have this time of year and get out for a look with your own two eyes.

This column originally appeared in the Grand Haven Tribune on 17 October 2008.

All us kids are back to school for real now. You know the routine: wake up, eat breakfast, go to school, come home, have supper, do homework, go to bed. Repeat.

Speaking of homework, one assignment that teachers often give in the fall is to keep a journal about the phases of the moon. This activity is perfect for this time of year, and it can be fun and interesting for both kids and parents.

I hope this short column gives you a few pointers to get started.

First you need to know that you don’t need much preparation to get started watching the moon. You don’t need an atomic clock, a telescope, and a GPS. You only need a notebook, a clock, and maybe a magnetic compass.

The compass is to find your bearings. Like the sun, the moon rises in the east, climbs highest in the sky above the southern horizon, and sets in the west. A compass will help you look for the moon in the right place, and keep track of where the moon was when you saw it.

The clock is so that you know when to look for the moon. Unlike the sun, the moon does not always rise and set at about the same time of the day. In fact, this is something that your teacher probably wants you to learn by watching the moon, so I won’t give too much away.

Here is one tidbit, however, that might help you find the moon day after day. The moon rises and sets about fifty minutes — nearly an hour — later each day compared to the day before. So if you saw the moon set around super time at 6 pm one day, look for it to set around 7 pm the next day, 8 pm the next, and so on.

You’ll need a notebook to keep track of your moon observations. Be sure to note the time you saw the moon, in which direction you looked to see it, and what the moon looked like. You can make a little drawing of the moon when you see it. Of course, follow your teacher’s instructions.

As you probably know, the moon looks different each time you see it. The moon goes through phases. Sometimes the moon looks like a crescent — a cookie with a large bite taken out of it. Sometimes it looks like a half moon. Astronomers call the half moon a quarter moon. When you learn about the moon phases you’ll know why. Sometimes the moon looks nearly full, a phase astronomers call gibbous. And sometimes the moon is full and bright.

It takes the moon about a month to go through all of its phases. By keeping track of the moon and its phases for a month or more you’ll learn on your own what makes the moon look the way it does.

Here are just two more things to keep in mind as you watch the moon this fall. First, the moon is often visible during the day. You don’t need to stay up late to see the moon. And second, no matter what you’ve heard or think about the phases of the moon, they are not caused by the earth’s shadow.

The only time the moon moves into earth’s shadow is during a lunar eclipse. Lunar eclipses are pretty rare. It’s too bad that we won’t see one this year or next.

A small lump of rock and ice 30 kilometers (about 20 miles) in diameter presently quietly drifting in the cold, dark reaches of the outer solar system, beyond the orbit of Uranus, is responsible for one of the nicest astronomical events of the summer.

Every year around the 12th of August the earth passes through a narrow stream of debris left in the interplanetary wake of this tiny icy nucleus, giving rise to a meteor shower that can produce hundreds of shooting stars per hour under ideal conditions.

The meteors are called the Perseids because the glowing streaks these shooting stars create as they flare in the upper atmosphere point back to a spot in the constellation Perseus.

Perseid meteors are sand-grain-sized flecks of matter shed by a periodic comet named Swift-Tuttle. The comet was discovered in July of 1862 independently by astronomers Louis Swift and Horace Tuttle. Comet Swift-Tuttle last passed near the sun and earth in 1992 and will not swing by again until 2125.

If you’ve never seen a shooting star, the next few nights will offer the best opportunities of the year. If you’re a shooting star aficionado, then you know the drill: dress warm, get comfortable in a dark location with a good view of the heavens, and keep your eyes to the sky.

Timing is important to take in the best of the Perseid meteor shower. The stream of debris left behind by comet Swift-Tuttle is a narrow one. Earth plows through the thick of it in just a couple of hours. Experts at the International Meteor Organization (IMO, on the web at http://imo.net/) predict the main peak of the Perseids between 11:30 and 14:00 Universal Time on Tuesday, Aug. 12. That is Tuesday morning between 7:30 and 10:00 am EDT (UT-4 hours), for us in West Michigan.

But don’t wait until then to catch the show. Shooting stars are best observed in a dark, dark sky. The darker, the better. So our best opportunity to view the Perseids this year is late Monday night into early Tuesday morning. The moon will be nearly full, but will be low on the horizon in the west after midnight, clearing the sky for the Perseids shooting stars.

If the sky is clear, why not pull an all-nighter? Have a nice look at the nearly full moon early in the evening, then watch the sky darken as the moon settles in the west. After midnight, check out Jupiter in the south and let the Milky Way stretch overhead as the Perseid meteor shower builds in toward dawn.

If the weather does not cooperate Monday night into Tuesday morning, or if you’d like to catch an encore performance, set out again Tuesday evening. There are predictions of a second and perhaps even a third Perseid peak this year that might make the shower persist longer than usual. But the nearly-full moon Tuesday night will be something of a spoiler.

The shower’s namesake, the constellation Perseus, this time of year rises above the horizon in the southeast an hour or so after sunset. Perseus is not a very conspicuous constellation. To find him, search for a pattern of stars that looks like a Greek God with a sword in one hand and the head of Medusa in the other. Use your imagination.

You don’t need to look toward Perseus to see Perseid meteors, anyways. Shooting stars will appear just about anywhere in the sky. But watch their streaks — Perseid meteors all seem to streak away from Perseus.

The International Space Station will be passing over the Tri-Cities area in the mornings and evenings over the next few days. If you’ve never seen the ISS, or if you haven’t seen it in a while, try to get out and take a look. The space station is bigger and brighter than ever, easily the brightest thing in the sky when the conditions are right.

NASA astronauts have taken to calling the ISS by the call sign or nickname Alpha.

ISS in June, 2008. Courtesy NASA.

Look for Alpha to rise out of the haze Friday 18 July in the west-southwest at 11:22 pm and pass almost directly overhead. But don’t be late, Alpha will be out of sight in the northeast by about 11:28 pm.

Alpha will be visible passing over our area several times on Saturday July 19, twice in the early morning hours, and twice in the evening. On these passes, it won’t be quite as bright as on Friday.

On Saturday morning, watch for Alpha in the northwest beginning at 4:10 am, then again, after one orbit, in the west at 5:45 am. On Saturday evening, watch for Alpha in the south-southwest beginning at 10:11 pm, then again in the west at 11:46 pm.

On Sunday 21 July Alpha will be making two very bright passes high overhead: in the morning, watch for it to rise in the west-northwest at about 4:33 am; in the evening, watch for it in the west-southwest at about 10:34 pm.

There are several websites you can turn to for updates and more detailed information about spotting the ISS and other satellites. The simplest is http://spaceweather.com/flybys/. You’ll need only a zip code to get up-to-date satellite flyby predictions. A more complete database is available at http://heavens-above.com/. To use heavens-above you’ll need to select your viewing location from a database, and, if you like, you can register as a user and save your preferences.

The ISS orbits the earth once every 91 minutes at an altitude just over 180 miles. To stay in orbit the station clips along at just over 17,000 mph.

The station doesn’t need motors to keep moving and stay aloft. Once a new station part is launched into orbit above earth’s atmosphere by a NASA space shuttle or a Russian or European Space Agency rocket, the part continues to coast along at the proper orbital speed.

The ISS does experience a slight bit of air drag in low-earth orbit, and because of this its orbit slowly decays. To keep the station aloft, visiting supply and servicing spacecraft give it a boost from time to time.

ISS Altitude. From Heavens-above.com.

Presently, the ISS is about 60% completed. It is now about the size of a football field, its profile is dominated by a number of large, shiny solar panels. The panels generate all of the space station’s electrical power and are the reason why — when sunlight glints off them just right — Alpha appears so bright.

The total weight that has been lifted into orbit to build the ISS now exceeds 600 thousand pounds. The volume of living space aboard the ISS is now about 15 thousand cubic feet, about the same as a two-thousand square-foot home (with eight-foot ceilings).

When you see Alpha pass overhead be sure to wave to the two Russian cosmonauts and one American astronaut that presently call it home. Commander Sergey Volkov and Flight Engineer Oleg Kononenko have been aboard since April, 2008 and Flight Engineer Gregory Chamitoff has been aboard since just last month when the space shuttle Discovery visited the station.

NASA astronauts Greg Chamitoff (left), Garrett Reisman, both Expedition 17 flight engineers; Russian Federal Space Agency cosmonauts Sergei Volkov, commander, and Oleg Kononenko, flight engineer.

Posted on 14 July 2008.

This week brings some of the best views of the planet Jupiter this year. Look low in the southeast after sunset to catch the solar system’s biggest planet at the start of its all-night transit of the night sky. After midnight, Jupiter will be above the southern horizon, and at sunrise he will be setting in the west.

Earth is presently between the sun and Jupiter. When earth and Jupiter are configured in this way, the two planets are as close as they can get, and we see Jupiter in the sky opposite the sun. Astronomers call this configuration “opposition.”

Technically, Jupiter was at opposition on July 9.

Although we are as close to Jupiter as we can be, we are still quite a ways away. The radius of Jupiter’s orbit is about 10 AU, with an astronomical unit (AU)—the average distance between the earth and sun, or the radius of earth’s orbit—as a unit of measure. Thus at opposition, Jupiter is about 9 AU from earth, or something a bit less than 840 million miles. It takes a ray of light—the fastest thing in the Universe—slightly more than an hour to travel between Jupiter and earth when Jupiter is at opposition.

Even thought Jupiter is relatively close and about 11 times the size of earth, you will need a small telescope to get a good view of the giant planet. But even a small, backyard scope will bring out spectacular detail.

You can easily see four of Jupiter’s many moons (the number stands at sixty three now) with a small telescope, or even with a pair of binoculars if you hold them steady enough. These moons are called the Galilean moons in honor of the Italian astronomer-scientist Galileo who first observed them in 1610. It is said that Galileo’s telescope produced a magnification of only 33 times.

Through a small telescope, Jupiter’s moons look like bright stars arrayed side to side next to Jupiter. But the moons orbit Jupiter pretty quickly, so the pattern they form changes nightly.

If you catch sight of the moons tonight, two will appear on one side of the planet and two on the other. By tomorrow all four will be arrayed out to one side. We can’t really see the moons’ orbits Jupiter because we see their orbits mostly edge on, so to us the moons appear to travel back and forth, behind and in front of Jupiter.

In fact, if you would like to watch for such a thing, one of Jupiter’s moons, named Io, the moon that orbits closest to Jupiter, will begin to move in front of Jupiter around 4:40 am Sunday morning. It would take some dedication to get up to watch this.

There is a nice Jupiter and Galilean moon simulator available online at http://www.shallowsky.com/jupiter.html.

Another thing to watch for on Jupiter through a small telescope is its so-called great red spot. Jupiter’s great red spot is a storm in Jupiter’s cloud tops that is larger than earth. The storm has raged for hundreds of years, at least. The spot appears slightly reddish, in contrast to Jupiter’s brownish natural hue, because the storm has mixed to the surface a different concentration of molecules.

Jupiter rotates rapidly, about once every 10 hours, so you have to look at the right time to catch sight of Jupiter’s great red spot. According to the online `Juplet’ noted above, the great red spot should be visible around midnight tonight, and again early Monday morning.

Jupiter

Hubble images of Jupiter, courtesy the Hubble Heritage website.

This weekend, the northern lights might possibly be visible from West Michigan.

A number of satellites and observatories that monitor the sun — the ultimate source of the northern lights — have recently detected solar activity that sometimes gives rise to the northern lights.

The northern lights, or aurora borealis as the phenomenon is called in scientific circles, are most common in the spring and fall. And while the aurora borealis is very common at northern latitudes, only especially strong displays are visible from the mid latitudes.

Last week, observers in Michigan reported seeing the northern lights.

Your best chances to see the aurora this weekend are late in the evening when the sky is very dark. You will need to find a place where you have a dark view toward the north. You will also need to be patient and maybe lucky, as the forecast calls for mostly to partly cloudy skies for the next few days.

I saw the northern lights for the first time when I was about 10 years old. We were on an epic family vacation, camping our way from Chicago to see the great places “out West” in a green Pontiac station wagon without air conditioning. The trip — with my two sisters and I sliding around on the vinyl seats of that hot wagon for hours on end, trekking relentlessly across the great plains and desert Southwest — brings back lots of memories, one of which is the night we saw the northern lights.

For some reason, I thought for a long time that the northern lights were caused by sunlight glinting off Earth’s icy north polar region. But now I know better.

Aurora is indeed produced by the sun, but not in the way I thought as a child. In addition to radiating light, the sun gives off in all directions an energetic stream of atomic-scale particles. These particles include electrons, protons and fragments of atoms called ions — the debris from the nuclear reactions and energy flows that make the sun shine. This matter is called the solar wind.

The solar wind is not a gentle breeze. It plows through space at about a million miles an hour with enough energy to cook those of us who live on Earth’s surface. And cooked we would be, were it not for the fact that Earth has a relatively strong magnetic field, which is an effective shield against the solar wind.

Earth’s magnetic field, which extends thousands of miles into space, deflects most of the charged particles that comprise the solar wind. But Earth’s magnetic field also traps some of the solar wind, and funnels the charged particles toward the north and south poles.

When the solar wind particles encounter oxygen and nitrogen atoms high in Earth’s atmosphere, the particles cause the atoms to glow. The stronger the solar wind, and the better aligned it is with Earth’s magnetic field, the brighter the glow and the more it extends from the polar regions.

The light given off by oxygen and nitrogen atoms is characteristically greenish and reddish. This is what we observe when we see the northern lights.

In some ways, I liked better the explanation I believed as a kid. But this is true of many things.

One thing is for sure — seeing the twisting shimmering glow of a bright display of the northern lights is a fascination that can, at least for a while, make you wonder like a kid again.

On the Web, aurora alerts are available from http://spaceweather.com.

In the early evening on Wednesday Feb. 20 the moon will slide through the shadow earth casts in the solar system giving us a chance to observe a total lunar eclipse.

The eclipse will be at least partly visible from locations covering about three quarters of the earth’s surface. The eclipse will not be visible at all for observers in Australia and northward through most of China, Japan and eastern Russia. Observers in the America’s have the best view; we will be able to see the eclipse from start to finish.

These diagrams are from NASA’s eclipse website. Much more information about this week’s eclipse, and about future eclipses, is available at NASA’s eclipse page.

The sun will set in West Michigan at 6:23 pm on the day of the eclipse. The full moon, however, in anticipation of its transit west to east through earth’s dark shadow will rise in the east a few minutes earlier, at 6:08 pm.

The eclipse will begin just after 7:30 pm when the moon starts to slip into the faint part of earth’s shadow called the penumbra. For the hour or so the moon moves in earth’s penumbra it will be difficult to see anything special is happening, but the situation will change at 8:43 pm when the moon will first encounter the dark part of earth’s shadow called the umbra. When the moon moves into earth’s umbra it will look as though Stephen King’s Langoliers are eating up the moon.

The moon will be completely in earth’s umbra for about 50 minutes between 10:00 and 10:51 pm. During this time, the moon will appear a dull, copper red, flanked by the planet Saturn to the left and the bright star Regulus in the constellation Leo to the right.

The moon will then slip out the other side of earth’s shadow, through the not-totally-dark penumbra and into the bright sunlight again by 1:17 am.

Take time to watch Wednesday’s eclipse if weather permits because while we have a good view of this one, we won’t be able to see another until December, 2010.

So what’s the difference between the umbra of earth’s shadow and its penumbra? You can see for yourself by conducting a simple experiment on a sunny day. Hold a ball in the sun so that it casts a shadow onto the floor or a wall. If you examine the shadow carefully you will see that its edge – the dividing line between bright and dark – is not perfectly sharp. Also, if you move the ball away from the wall you will see that the dark part of the shadow gets smaller, as you probably expected, but that its edge gets fuzzier.

The totally dark part of a shadow is its umbra. The umbra of a sun shadow is the region of space behind a shadow-casting object that light from the sun cannot reach. Consider the situation from a different perspective: an observer in the umbra of a sun shadow is not able to see the sun at all.

The fuzzy part of a shadow is its penumbra. The penumbra of a sun shadow is the region of space behind a shadow-casting object that some, but not all, rays of the light from the sun can reach. An observer in the penumbra of a sun shadow is able to see part, but not all, of the sun.

Earth’s shadow at the distance of the moon is really fuzzy. The umbra is about 2.5 times the diameter of the moon and the penumbra is almost 5 times the diameter of the moon. When the moon is in the penumbra it looks normal, but just a bit dimmer than usual. When the moon is in the umbra it looks very, very dark.