Tag Archives: ecliptic

Mercury, Venus and Saturn align in the morning sky

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Mercury, Venus and Saturn – along with Spica, which is similar to Saturn in brightness – will appear together in the southeastern sky at dawn. The maps show the scene about one hour before sunrise. Maps created with Stellarium

The upcoming Mercury-Venus-Saturn alignment, while not a real conjunction, is sure getting a lot of attention thanks to an inaccurate illustration on the Web depicting the three over the Pyramids of Giza. The planetary trio will be approximately equidistant and span an angle of 14 degrees (equal to 1 1/2 fists extended at arm’s length against the sky) on Monday morning Dec. 3. They’ll also appear in virtually the same lineup the mornings before and after.

When astronomers refer to a conjunction of two or more celestial bodies, they usually mean the objects are lined up closest together north-south of one another. When due south and highest in the sky, two planets in conjunction are stacked on top of each other. There are also conjunctions in longitude, when two celestial objects are closest together east-west or side-by-side.

Since none of the three planets is in conjunction, the Dec. 3 gathering is simply an interesting alignment. You’ll also notice from the diagram that for the next few mornings their configuration changes little. Tomorrow, Venus and Saturn will be a bit closer than Venus-Mercury; on Monday they’re equidistant and on Tuesday, Venus is closer to Mercury than Saturn.

The word planet comes from the Greek aster planetes or “wandering star”. And that’s exactly what they do. Ancient peoples thought they were carried around the Earth on invisible crystalline spheres. Today we know the truth – each orbits at a different distance from the sun with a period that depends on that distance. Mercury is closest and orbits fastest at 88 days; distant Neptune requires 165 years.

The ecliptic is the path followed by the planets, moon and sun across the sky. This narrow “highway” is a consequence of the planets orbiting the sun in nearly the same flat plane.

Planets are frequently in conjunction because they all follow the same path across the sky called the ecliptic.

It follows that if you’re all driving the same highway at different speeds, sooner or later two or more planets will pull up alongside each other. From our perspective, they’ll appear close together in the sky.

And since the planetary highway is approximately circular like a racetrack, interesting gatherings or conjunctions happen repeatedly over the months and years.

So if you combine the planets’ varying speeds according to distance, their shared “roadway” and our changing perspective on them as Earth revolves around the sun, you’ll understand why the current morning planet alignment will soon be broken.

Venus and the crescent moon about an hour before sunrise on Dec. 11.

No need to feel like you’re missing the opportunity of a lifetime though. Since these wandering stars are ever on the move, there’s an kaleidoscopic supply of conjunctions and alignments of every kind.

The really exciting ones usually involve the brightest planets or a planet-moon combo like the Jupiter-moon conjunction this past Wednesday.

Playing around with the free star-charting program Stellarium, I found that the next bright conjunction happens soon enough. On Dec. 11, the very thin crescent moon will be only 1.5 degrees south of Venus at dawn. It’s Christmas all year round when it comes to presents from the sky.

Shine on Harvest Moon for me and my gal

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The Harvest Moon from a few years back rises over Lake Superior at Brighton Beach in Duluth, Minn. Photo: Bob King

Autumn blew in last weekend with a chill and flourish of color. This weekend we get fall v2.0 with an appearance by the Harvest Moon, the full moon closest to the autumnal equinox.

The Harvest Moon harks back to our agrarian past when farmers could continue harvesting by its light for several evenings in a row. Because the angle of the full moon’s path to the horizon is very shallow in September and October, the time difference between successive moonrises is only about 20-30 minutes instead of the usual 50-60. With moonrise happening on the heels of sunset, farmers could harvest their crops into the night in the days before electric lighting.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King

The moon, planets and sun all travel along the ecliptic, an invisible circle in the sky that defines the plane of the solar system. During early fall, the ecliptic runs nearly parallel to the eastern horizon for the northern hemisphere. As the moon scoots eastward along this path at the rate of one fist held at arm’s length each day, its rising times vary by a half hour or less. For several nights in a row a big moon seems to rise at nearly the same time – a constant companion of sorts.

Exactly the opposite happens 6 months later in spring, when the moon’s path is tipped at a steep angle to the horizon. While it moves the same amount each night – one outstretched fist – the moon is much farther below the horizon on successive nights, delaying moonrise by an hour or more. You’ll see the full moon one night and wonder why it’s taking so long to rise the next.

We all love to see a big orange moon on the horizon. If you want to know when it’s coming up in your neighborhood this weekend, go to the U.S. Naval Observatory website , select the link (data for the sun and moon for one day or the whole year) and specify your location. When the table pops up, be sure to add an hour for Daylight Saving Time to the times shown. For Duluth, Minn. the full moon rises at 6:21 p.m. tomorrow night (Sept. 29) or 30 minutes before sunset, 6:46 p.m. on Sunday and 7:14 p.m. Monday. Happy gazing!


The title of this blog refers to Shine On, Harvest Moon, a Tin Pan Alley song from the early 1900s. Click the video to see and hear it sung by two of my favorite comedians – Laurel and Hardy.

Full Red Moon of August is Blue too

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The nearly full moon grazes the treetops during twilight last night July 31. Tonight it will be fully illuminated by the sun and shine brightly in the constellation Capricornus. Photo: Bob King

With 31 days in August and a full moon happening tonight, we’re guaranteed a second full moon at the end of the month. A fine pair of bookends these. Full moons are often known by multiple names; the two most common for August are the Sturgeon Moon and Red Moon. We’ll need them both this month.

The first refers to the good sturgeon fishing this time of year for Indians living in the Great Lakes region. “Red” comes from the color the moon often appears during the summer months, when haze, humidity and low altitude conspire to change its tint from white to ruddy. The second full moon in a month is also called a Blue Moon. Blue Moons happen about once every two or three years – this one occurs on the 31st. That means this month’s Red Moon will also be Blue!

July’s full moon in Sagittarius was about as low as a full moon gets. Tonight the moon will be in Capricornus and noticeably higher up (almost three fists) compared to last month. Created with Stellarium

Does the full moon seem higher in the sky this month than July? It ought to. Last month’s moon was in the constellation Sagittarius the Archer, the same place the sun occupies during the month of December. It strode low across the southern sky peaking in the south a little more than two “fists” above the horizon here in Duluth, Minn. At the same time, the sun was in Gemini and as high in the sky as it gets for the year.

When we look at the full moon the sun is at our back. That’s why the moon appears opposite the sun in the sky, rising when the sun sets. Illustration: Bob King

Since that time, the sun has slid southward (lower) along its yearly path called the ecliptic as the season mellows to late summer. Because the full moon is directly opposite the sun and follows the same path, it moves northward or higher in the sky to keep pace with the sun’s decline in the south.

That’s why it’ll be higher in the sky tonight compared to June and July. The pattern continues until the winter solstice, when the full moon glares hard and white overhead in Gemini, while the sun shines meekly 180 degrees away in Sagittarius.

The sun’s path, shared by the moon and planets, is called the ecliptic. Because Earth’s tilted axis is tilted 23.5 degrees, the sun swings high and low along the ecliptic with the changing seasons. Click image to see an animated version. Credit: Durham University

If you’re game to watch a moonrise, click HERE to find when it cracks the horizon for your town tonight. Whatever the color, enjoy the sight.

Meet the ecliptic, one of scariest things in astronomy

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About 20 minutes after sunset watch for the pair of Venus and Mercury (below Venus) low in the southwestern sky. Maps created with Stellarium

Planet watchers with a clear view to the west-southwest can find Venus and Mercury tonight through next week. Venus is much brighter and easier to see and located about five degrees (three fingers held together horizontally at arm’s length) in the southwest about 20 minutes after sunset. It’s well to the left of the bright glow along the horizon that marks the location of the sun. Mercury is directly below Venus and nearly lost in the twilight. You might be able to see it with your naked eye, but bring binoculars just in case.

Mercury reaches greatest angular distance from the sun Monday. As seen from Earth, it's well off to the left or east of the sun at dusk. Telescopes will show a "half-moon" phase. Credit: ESO

Mercury reaches greatest elongation from the sun this coming Monday. Normally, when it’s so widely separated from the sun, we can see it with relative ease at dusk. But because the angle of the ecliptic – the path the planets, sun and moon follow -  is so shallow in the western sky in November, Mercury nearly scrapes the horizon. Venus does only a little better.

Mercury and Venus travel along the ecliptic, which is tipped at a very low angle in the western sky in November. For southern hemisphere observers, the ecliptic is shifted up and to the right, making a steeper angle with respect to the horizon.

Southern hemisphere observers see Venus and Mercury very differently this month. Just as the season are reversed, at southern latitudes, the ecliptic is tipped up at a steep angle and Mercury is easily visible at dusk. Mercury is best visible during evening hours for northern hemisphere observers during the spring months, when the ecliptic stands high in the west.

The ecliptic is the big circle the sun moves along in the course of a year. Also shown are the celestial equator - an extension of Earth's equator into space - and the north and south celestial poles, extensions of our axis into space. Credit: Joshua Cesa

Notice in the diagram above how the sun lies exactly on the ecliptic. That’s because this imaginary circle is defined as the plane of the Earth’s orbit around the sun. Since the sun’s movement is really just a reflection of Earth’s orbital motion, the ecliptic can also be viewed as the path the sun takes through the sky in the course of a year. And that path is a big circle inclined to our planet’s equator by 23.5 degrees.

Why 23.5? That’s the tip of Earth’s axis. That tip is projected on the sky just like our orbit. It’s what causes the sun to appear high in the sky in summer and low in winter.

Each day the sun moves about one degree or two full moon diameters eastward in the sky. Of course the sun’s not really moving. The Earth, speeding along its orbit at 18 1/2 miles per second, travels 1.6 million miles a day. From our ever-changing perspective, the sun appears to slowly slide east along the ecliptic.

Now you might be wondering why the planets and the moon also drive the same highway as our planet does. Turns out their orbits are tipped only a little bit with respect to Earth’s. In other words, all eight planets essentially lie in the same flat plane. As each orbits the sun, they follow the same familiar ecliptic path and pass through the same dozen zodiac constellations.

Mercury’s orbit is tipped the most with respect to the ecliptic at about 7 degrees. That’s why it’s farther than Venus (3.4 degrees) as shown in the ecliptic illustration above. Uranus varies least with an inclination of just 0.8 degrees.

Because the moon's orbit is tilted 5.2 degrees (exaggerated for effect here), it only lines up to create eclipses when it's passes through the ecliptic at new and full moon. Credit: C.R. Nave, Georgia State University

The word ‘ecliptic’ derives from eclipse. When the moon, which has a slightly tipped orbit, intersects the ecliptic at full moon phase, the sun, Earth and moon are exactly lined up and a lunar eclipse occurs. When it intersects at new moon, the sun, moon and Earth (in that order) are exactly lined up and a solar eclipse occurs.

It’s fascinating how the interplay of orbits creates so many fun events to watch for in the sky. Understanding the ecliptic isn’t always easy – I’ve seen a few pairs of eyes glaze over -  but once you do, you’ll be on the path to astronomical enlightenment.

Speaking of eclipses, the next one’s coming up very soon! A total lunar eclipse will occur in the early morning hours of December 10 for the western half of the U.S., Canada, Australia and central and eastern Asia. I’ll provide more details and viewing tips as we get closer to that date.

A little tilt is all it takes

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I hope the first couple days of spring have lived up to seasonal expectations where you live. They have here, but now a howling snowstorm is on its way, threatening to repave all the lovely, exposed ground with a foot of wet, white concrete.

The sun is shown at noon on the first day of spring (left) and 10 days later. The arrow shows the direction it's moving. Notice that the sun's path, called the ecliptic, intersects the imaginary arc of the celestial equator on the first day of spring. By the 30th, the sun has moved noticeably north of the equator and is higher in the sky. Created with Stellarium

In Sunday’s blog we discussed that the reason for the seasons has to do with the tip or inclination of Earth’s axis, rather than our planet’s distance from the sun. The tip causes first one hemisphere and then the other to alternatively face toward and away from the sun. This makes the sun’s apparent path across the sun swing from low to high to low in the year-long cycle of four seasons. Spring began when the sun crossed the imaginary projection of Earth’s equator – called the celestial equator – on its march northward.

This diagram shows the various tilts of the planets and dwarf planet Pluto. Credit: Image courtesy of Courtney Seligman / cseligman.com

What about the other planets in the solar system? Do they have seasons too? Well, to have seasons, you need to be tipped on your side. You can see that Mercury, Venus and Jupiter have little to no axial tip, so it follows they don’t experience the seasons. The sun follows the same path across the sky, neither dipping low nor ascending to the heights, the entire year round on these planets.

On Earth, and sometimes in Duluth, liquid water returns as winter transitions to spring. Photo: Bob King

Venus is a bit of an oddball. It’s tipped so far over – 177 degrees – that its north pole is where the south pole should be. This extreme tip means Venus is actually rotating backwards or clockwise as seen from above the plane of the solar system. From the same perspective, most of the other planets rotate counterclockwise.

Because Earth, Mars, Saturn and Neptune have similar inclinations, they all experience similar seasons. For instance, right now it’s late spring in Mars’ northern hemisphere and early spring in Saturn’s northern hemisphere.

Keep in mind that seasons on these outer planets last longer than those on Earth, because they take longer to orbit the sun. A typical season on Mars last twice as long as the equivalent on Earth. Neptune takes takes 165 years to journey once around the sun, making spring a tedious 41 years in length. Well, maybe you like spring more than I do.

Pluto and Uranus also have seasons, but surprising things happen when you’re rolling around the sun on your side.

The Hubble Space Telescope photographed Uranus and its rings between 2003 and 2007. The rings, which lie above the planet's equator, went from open presentation in 2003 to edge-on in 2007. Credit: NASA/ESA/M. Showalter

Uranus orbits the sun every 84 years. As the north polar regions face the sun for 21 years, the sun hugs the horizon at the equator and the south pole is in complete darkness. During the next 21 years, the equator faces the sun and all parts of the planet experience sunlight. The poles see sunlight too, but just barely, as the sun circles the horizon for years on end. This is followed by 21 years of sunlight at the south pole and an equally long night at the north pole. The Uranian year wraps up with the equator once again facing the sun with sunlight across the whole planet. Just weird.

Seasons on Earth keep life interesting, but when you add in the rest of the planets, you really get a feel for how much variation is possible from a simple little thing like tilt.

Venus proves that beauty can be elusive

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Venus is the solitary light above the TV and radio towers in Duluth's "antennae farm" last night. Details: 200mm lens at f/2.8, 1/80-second exposure, ISO 800. Photo: Bob King

Heck, it’s the brightest planet, and about as far as it gets from the sun in the evening sky, so how come Venus is hiding in the sunset glow? Checking the stats, I see Venus reached its greatest eastern elongation of 46 degrees – largest distance east of the sun – only a week ago. That’s half the distance between the horizon and the overhead point. Shouldn’t the planet pop right out in the sky?

Nope. Instead, you’ll find Venus very low in the southwestern sky well to the left of the sunset point about a half hour after sunset. If you’re on a lake or standing in an open field, it’s easy to see, but for many of us who live around buildings and trees, it’s easily lost behind a roof line. This is sad, because we can’t fully appreciate how amazingly bright this planet is so long as it can’t step up to the plate.

Venus isn’t to blame. It has no choice but to follow the path all the planets must follow as they orbit the sun. Called the ecliptic, it’s an imaginary circle in the sky that closely defines the flat plane of the solar system. Since all the planets (and moon) orbit in or near this plane, we always see them following the same path through the same 12 zodiac constellations over the days, months and years. Even the sun tracks along the ecliptic as seen from Earth, because we’re driving on the same race track as the rest of the planets. Since we’re stuck on Earth and can’t see ourselves traveling along the ecliptic from afar, the sun appears to follow the ecliptic instead. Watching the sun’s seasonal travels up and down the sky is a reflection of our planet’s orbital motion around the sun. The sun’s not moving, we are.

The changing angle the ecliptic makes to the horizon strongly affects how easily planets are visible when near the sun. The left panel shows Venus tonight when its path follows a long angle to the horizon; the right panel depicts its next period of evening visibility in the spring of 2012, when it will be much higher. Created with Stellarium

This time of year, the ecliptic makes a very shallow slant to the evening horizon for sky watchers in mid-northern latitudes. Venus may be a long ways to the left of the sun, but because its path is at a very low angle, it never gets far from the horizon. That’s why we have to work a little more to find it.

Every couple years, Venus puts on an evening show in the early spring sky. The next time this happens is March 2012. During spring, the ecliptic cuts the horizon at a much steeper angle. While Venus will be the same angular distance from the sun then as it is tonight, the steep slant lofts the planet high into the sky where its brilliant beacon can be seen by all.

In a sense, Venus is a bellwether for where the sun is headed next. The sun reached its highest point in the sky (farthest north) for the northern hemisphere on the June 21st summer solstice. Since then it’s been dropping to the south and sinking lower in the sky every day as we approach fall. Notice that Venus is to the east of or ahead of the sun. Where the planet is now is where the sun will be in late September as its heads southward to its lowest point in the sky called the winter solstice. In spring, Venus presages the sun’s ongoing northward climb back to its high point at the summer solstice. That’s why the planet is much higher and easier to see.

Because Venus orbits between the sun and Earth, we get to see go through phases like the moon as the angle between it, us and the sun varies during Venus 225-day long orbit. Illustration: Bob King

As I described in a recent blog, telescope users can see Venus’ phase if they look at the planet during early dusk before it gets too low for a sharp view. Its currently a “half moon” as seen from our perspective here on Earth. Over the coming weeks, Venus will appear to get closer and closer to the sun at the same time as its phase changes from half to crescent.

I wish you well in your search for this somewhat elusive planetary goddess. If you succeed in finding her, please let us know.