Who’d a- thunk it? Mercury May Have Meteor Showers Too

Artist’s concept of Mercury crossing the debris trail of Comet Encke, sparking a recurring meteor shower. New evidence from the MESSENGER mission suggests the planet receives regular doses of Mercurial dust. Credit: NASA’s Goddard Space Flight Center

Of course, of course, it only makes sense. We’re so caught up in watching meteor showers on our own planet, who ever thinks about meteors at Mercury? Or Venus for that matter? This week NASA announced that regular spikes in the amount of calcium in Mercury’s upper atmosphere bespeak a cyclical source. The likely culprit? Comet 2P/Encke.

Like breadcrumbs dropped to mark a path, dust and fragile bits of rock are released through vaporization of a comet’s ice and pushed back by the pressure of sunlight to form a tail. The larger pieces are left behind to fan out along the comet’s orbit. If by good fortune Earth’s orbit happens to intersect the debris trail, we see a shower of meteors in the sky.

This photo, made by NASA’s Spitzer Space Telescope in infrared light, shows Comet Encke’s glowing nucleus/nuclear region and a trail of warm dust and pebbly debris shed by the comet along its orbital path. Credit: NASA

Most recently, the Geminids put on a great show, although their origin lies with the peculiar rock-shedding asteroid 3200 Phaethon.

Comet 109P/Swift-Tuttle brings us the familiar Perseid meteor shower, while 2P/Encke gives rise to several meteor streams - the Southern and Northern Taurids, showers that peak in October and November, and the daytime Beta Taurids in June and July.

Measurements taken by MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer have revealed seasonal surges of calcium that occurred regularly over the first nine Mercury years (1 year = 88 Earth days) since MESSENGER began orbiting the planet in March 2011. Just as we saw huge spikes in the amounts of metals like magnesium and iron in Mars’ upper atmosphere during Comet’s Siding Spring’s brush with the planet last October, MESSENGER’s instrument detected periodic spikes in the amount of calcium – although with a twist.

A color- enhanced view of Mercury, assembled from images taken at various wavelengths by the cameras on board the MESSENGER spacecraft, shows a cratered composed with a surface composed of a variety of minerals. The circular, orange area near the center-top of the disk is the enormous Caloris impact basin. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Mercury’s has only a whiff of an atmosphere, what astronomers term an exosphere, the last thing you could call an atmosphere before encountering the vacuum of space. The shower of small dust particles peppering interplanetary space pass right down to the surface and strike the planet’s rocks, knocking calcium-bearing molecules free from the surface, which are then free to rise to greater heights. This process, called impact vaporization, continually renews the gases in Mercury’s exosphere as interplanetary dust and meteoroids rain down on the planet.

These type of impacts happen all the time, but scientists noticed a pattern in the calcium spikes that pointed to a repeating source. Sounds like a perfect time for a comet to step in. Examination of the handful of comets in orbits that would permit their debris to cross Mercury’s orbit indicated that the likely source of the spikes was Encke.

The Jupiter family of comets were all once long-period objects in the Kuiper the orbits of which were changed to short-period by close passes by Jupiter. The green circle is Jupiter’s orbit, the purple is Earth’s. Notice that when farthest from the Sun, the comets about as far as Jupiter is from the Sun. Credit: Wikipedia with additions by the author

“If our scenario is correct, Mercury is a giant dust collector,” said Joseph Hahn, a planetary dynamist in the Austin, Texas, office of the Space Science Institute and coauthor of the study. “The planet is under steady siege from interplanetary dust and then regularly passes through this other dust storm, which we think is from comet Encke.”

To test their hypothesis, Han and crew created detailed computer simulations and discovered that the MESSENGER were offset from the expected results but in a way that made sense due to changes in Encke’s orbit over time from the gravitational pull of Jupiter and other planets.

Pantheon Fossae – The striking troughs of Mercury’s Pantheon Fossae, the feature that MESSENGER scientists first called “The Spider” when they discovered it. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Comets get nudged by planets routinely, especially if they pass near Jupiter, the outer Solar System’s gravitational goliath. Jupiter, with the help of Neptune, has re-worked the orbits of countless bodies that once resided in the distant Kuiper Belt into shorter-period comets that swing around the Sun in 20 years or less. Called the Jupiter-family, there are about 400 known and Encke is one of them with an orbital period of just 3.3 years.

Who knows how many other meteor showers might pepper Mercury in a year, but scientists will be looking for potential signs of them in planet’s atmosphere in the months ahead. While they may not leave bold streaks of light as they do on Earth, they create something almost as amazing – a shower of particles that goes up instead of down.

Spectacular meteor storm lights up Mars during recent comet flyby

On October 19, when Comet C/2013 A1 Siding Spring flew just 87,000 miles from Mars, dust from its tail set the sky aglow with a meteor storm. This illustration is my feeble attempt to show what you might have seen standing on Mars next to the Curiosity rover at the time. Credit: NASA (background) with additions and changes by Bob King

Oh, to have stood under the Martian sky on October 19th! As Comet Siding Spring passed just 87,000 miles (140,000 km) from the planet that night, dust in its tail slammed into the Martian atmosphere at 126,000 mph, burning up in storm of meteoric madness. “Thousands per hour fell,” said Nick Schneider, instrument lead for NASA’s MAVEN Imaging Ultraviolet Spectrograph. It must have looked like those classic illustrations of the 1833 and 1866 Leonid meteor storm back here on Earth.

Composite image of Comet Siding Spring and Mars taken by the Hubble Space Telescope. The images have been added together to create a single picture to illustrate the true distance or separation (1/20th the apparent size of the Full Moon) between the comet and Mars at closest approach.  Credit: NASA/ESA

I participated in a teleconference yesterday with principal investigators for the instruments on the Mars Reconnaissance Orbiter (MRO), MAVEN and Mars Express spacecraft pressed into service to study Comet Siding Spring during its historic flyby. The comet is a visitor from the faraway Oort Cloud, a spherical repository of billions of icy comets up to 1 light year from the Sun. Some 4-5 Oort Cloud comets swing through the inner solar system every year; this is the first one we’ve ever studied up close. It was discovered at Siding Spring Observatory in Australia by Robert McNaught on January 13, 2013.

NASA’s MAVEN uses its IUVS to perform a scan of the Martian atmosphere along its limb. Scans found enhanced levels of metals from vaporizing comet dust. Credit: NASA

“Dust slammed into the atmosphere and changed the chemistry of the upper atmosphere,” said Jim Green, director, Planetary Science Division, NASA Headquarters in Washington. Data from MAVEN’s UltraViolet Spectrograph (IUVS), which scans of Mars’ upper atmosphere in UV light to determine its chemical makeup, saw big spikes in the amount of magnesium and iron during the flyby. These elements are commonly found in meteorites.

Before and after scans by MAVEN. At left is a profile of the atmosphere before the comet’s arrival showing carbon dioxide and other gases; at right is during the comet’s pass. Check out that huge spike to the right – that from magnesium. Elevated levels to the left indicate iron. Credit: NASA

Siding Spring turned out to be much dustier than expected, prompting Green to later add: “It makes me very happy hid them (spacecraft) on the backside of Mars.” “It really saved them. Even one well-placed hit from a high-speed dust particle could damage an instrument, and Siding Spring peppered the Martian atmosphere with “several tons” of dust.

MAVEN used its mass spectrometer – an instrument that identifies elements by how much mass they have – to record a big enhancement of the elements magnesium, manganese, iron and others from comet dust in Mars’ atmosphere. Credit: NASA

Meanwhile, MAVEN’s Neutral Gas and Ion Mass Spectrometer (NGIMS), picked up major spikes in 8 different metals from ablating comet dust including sodium, magnesium, iron and nickel. Jim Green pointed out that the increase in sodium may have tinged the twilight sky with a yellow glow. That and a recent increase in the amount of dust in the atmosphere over the Curiosity rover site may be the reason the comet was so difficult to photograph from the ground.

Only hours after Comet Siding Spring’s closest approach, dust particles hitting air molecules on Mars formed a temporary ionized (electrified) layer in its lower ionosphere 50-60 miles high. Credit: ESA

So we have a very dusty comet, a big meteor storm, the atmosphere spiced up with metals from burning dust.

Anything else? Heck, yes. The European Space Agency’s Mars Express Orbiter used its radar to send out radio waves of very low frequency down through Mars atmosphere to record the state of the ionosphere, a rarified layer of air between 60-250 miles (100-400 km) high. At the comet’s closest approach, the ionosphere was normal, but 7 hours later, impacting dust had created a brand new, temporary ionization layer.

Close-ups pictures taken by the Mars Reconnaissance Orbiter of Comet Siding Spring around the time of closest approach to Mars. They show the combined light of the tiny nucleus and much larger coma or comet atmosphere. Comet dust / rocks range in size from 1/1000 of a millimeter to 1 centimeter (~1/2-inch). Credit: NASA

The high resolution camera on the MRO photographed brightness variations in the comet’s light, nailing down its rotation period to 8 hours. But size-wise, we’re a little less clear. Estimates for the comet’s nucleus range from 984 feet to 1.2 miles (300-m to 2 km). For comparison, Comet 67P/Churyumov-Gerasimenko, currently orbited by Europe’s Rosetta spacecraft, is 1.5 miles (2.4 km) across.

Color variations in this photo by CRISM indicate different sized dust particles being ejected by the comet. Credit: NASA

Yet another instrument named CRISM (Compact Reconnaissance Spectrometer for Mars) made some intriguing measurements of the coma showing distinct differences in color – red here, blue there – indicating the comet is blowing out dust particles of different sizes.

As scientists continue to analyze the data collected by the fleet of space probes, we’ll see more papers and results soon. For now, the rare opportunity to study a comet up close from another planet was an unqualified success. You can listen to the replay of the hour-long conference HERE.

Seeing meteors? Delta Aquarids peak this week!

Meteors from Delta Aquarid meteor shower radiate from near the star Delta Aquarii not far from the bright star Fomalhaut in the Southern Fish low in the south before dawn. Stellarium

With the Southern Delta Aquarid meteor shower peaking tomorrow morning, the summer meteor season’s officially underway. While not a spectacular shower from mid-northern latitudes, why not chance  a look anyway. With a rate of 10-15 per meteors an hour from a dark sky you’re bound to catch at least a few.

The farther south you live, the better it gets. Observers in the southern hemisphere can expect double that number because the shower’s radiant will be much higher in the sky. Any meteors flashing south of the radiant won’t get cut off by the southern horizon like they do further north.

The annual shower gets its name from Delta Aquarii, a dim star in the dim zodiac constellation Aquarius. You don’t need to know the constellations to enjoy a meteor shower but it doesn’t hurt to know the general location of the radiant, the point in the sky from which the meteors appear to radiate. If you can trace the path of a meteor backward toward Aquarius, chances are it’s an Aquarid.

A Southern Delta Aquarid meteor captured on July 30, 2013. Credit: John Chumack

There are actually two meteor showers in Aquarius active this time of year – the northern and southern Delta Aquarids. The northern version sports fewer meteors and peaks in mid-August.

The Southern Deltas peak over the next two mornings – July 29 and 30. Both serve as warm-ups for the upcoming Perseid meteor shower that climaxes on August 12.

Tonight’s shower will suffer no interference from moonlight, making this an ideal time for meteor watching. Unfortunately, Perseid rates will be reduced by a bright waning gibbous moon. Don’t be surprised though if you see a few Perseids while you’re out. The shower’s just become active. If you can draw a meteor’s trail back to the northeastern sky, it just might be one. Perseids are also known for leaving bright streaks in their wake called trains.

Nearly all meteor showers originate from clouds of sand to seed-sized bits of debris spewed by vaporizing comet ice as they swing near the sun. The Delta Aquarids may trace its origin to dust boiled off Comet 96P/Machholz.

The best time to watch the shower is in the early morning hours before dawn when the radiant rises in the south-southeastern sky above the bright star Fomalhaut. Try to get away from city lights. Point your lawn chair south and spend some time in heavenly contemplation as you wait for Aquarius to toss a few javelins of light your way.

Catch some Quadrantids! Year’s first meteor shower peaks tomorrow morning

Watch for the Quads to radiant from a point in northern Bootes below the handle of the Big Dipper tomorrow morning. Stellariu

Tomorrow morning before dawn will be the best time to catch the very first meteor shower of the new year. The Quadrantids, named for the one-time constellation Quadrans Muralis, located below the handle of the Big Dipper, is one the most reliable showers of 2014. While the constellation’s now obsolete and erased from modern sky maps, its name lives on in fiery sparks of meteoric light. Pity the shower rains down at the coldest time of the year.

Ideally, you might see from 60-200 “Quads” per hour, when the shower’s very sharp peak coincides with the radiant being high in the northeastern sky for your location. This year the peak happens around 1:30 p.m. January 3, not exactly ideal for North and South American observers but perfect for folks living in the Middle East and Russia. Numbers will probably be less than half that for western hemisphere skywatchers.

Quadrans Muralis represents the wall quadrant, a instrument once used to measure star positions. Credit: Johann Bode atlas

Don’t let it bring you down. We’ll still see a good show tomorrow morning between the hours of 2 and 6 a.m as meteors emanate from the shower radiant located about a fist below the end of the Big Dipper’s handle in the northeastern sky. No moon will cast its glare to compromise the view. I’d suggest dressing warmly and relaxing in a sleeping bag on a reclining chair. Face to the east or north for the best view. The later you’re up, the more Quads you’ll see as the clock ticks closer to the daytime shower maximum.

A beautiful Quadrantid meteor captured over Duluth near the Big Dipper (partly outlined) a couple years back. Credit: Stephen Bockhold

Quadrantids are slower than other major showers like the August Perseids and December Geminids with speeds around 25 miles per second (41 km/sec). You’ll know you’re seeing a Quad if you can trace its trail back to the northeastern sky below the Dipper.

Peter Jenniskens, senior research scientist at NASA’s SETI Institute, traces the Quads’ origin to the asteroid 2003 EH1, a likely extinct or occasionally active comet. Its orbital characteristics agree well with the paths of our cold weather friends.

Rare Andromedid meteor outburst happening this weekend

Sketch of Comet Biela made by E. Weiss when it returned split in two during its 1846 appearance.

Ever heard of the Andromedids? They were briefly a major meteor shower – meteor storm might be a better description – visible in the late 1800s after the break-up of Comet Biela. The comet was discovered in 1772 and in 1805 but no one knew it was the same object until it was found for a third time in 1826 by Wilhelm von Biela and an orbit calculated.

The 1872 “Bielids” resembled the incredible Leonid meteor storm of 1833, an engraving of which is shown here.

When Comet Biela returned in 1846, astronomers were surprised to see it had split into two comets traveling side by side. Biela came back for another spin in 1852, but after that mysteriously disappeared until November 27, 1872. That night, instead of a comet, skywatchers were treated to a spectacular meteor storm from Biela’s dusty remains. Rates reached 3,000 meteors per hour.

Amazing displays of “Bielids” (renamed Andromedids) occurred again in 1885, 1892 and 1899. Since then, no one saw hide nor hair of Biela’s showery remnants until 2011, when the Andromedids returned with a rate of 50 per hour.

Kelly Beatty, who writes for Sky and Telescope magazine, passed along the news this evening that meteor specialist Peter Brown, using data from the Canadian Meteor Orbit Radar, has recorded a big outburst of Andromedid meteors in the past 24 hours. Comet Biela’s bits of dust are small and slow moving compared to most, striking Earth’s atmosphere at “only” 12 miles per second (19 km/sec).

Watch for the Andromedids to shoot from a point in the sky near the star Gamma in the M-shaped constellation Cassiopeia located high in the northern sky during early evening hours. This map shows the sky at 8 p.m. local time in early-mid December. Stellarium

Maybe you even saw a few when you were out watching the northern lights last night. Brown suggest that the shower has yet to peak, so tonight might offer an even better meteor show. Pity it’s overcast here in Duluth, Minn. or I’d be running off to the country with coat and camera.

If it’s clear by you, please take a look. We’d love to hear if the shower’s still in progress. The Andromedid radiant – the point from which the meteors will appear to radiate – is currently located near the center of Cassiopeia as shown in the map and up most if not all night for observers at mid-northern latitudes. December’s shaping up as a great time for meteor watching with the Geminid meteor shower set to peak next Saturday morning.

Click HERE for more on Biela’s Comet and HERE for additional good material about the Andromedids.

Bright moon dings Monday’s Orionid meteor shower

The Orionids are a reliable if minor meteor shower that returns every year in late October. Despite moonlight, keep watch for the occasional bright Orionid shooting from above the constellation Orion near the bright planet Jupiter on the mornings of Oct. 21 and 22. This map shows the sky facing south around 5-5:30 a.m local time.  Stellarium

Darn moon. We love its radiance but sometimes it just gets in the way. Count on it being there Monday morning Oct. 21 during the peak of the annual Orionid meteor shower. The shower is best seen in the wee hours before dawn when the radiant point, the spot in the sky from which the meteors will all appear to radiate, is high in the south above the upraised arm of Orion the Hunter.

Composite of many Orionids as recorded by Ohio amateur astronomer John Chumack

Normally we’d expect to see up to 20 very swift meteors per hour but moonlight will halve that number. Don’t let that stop you. The sun rises late, so you can squeeze in a little meteor watching between 5 and 6 before work.

Use the morning to dig out that old telescope and check in on Jupiter and the wonderful display of lunar craters along the moon’s terminator, the dividing line between day and night. I usually multi-task during meteor showers, poking the scope around from this to that while taking breaks for meteor-watching.

The Orionids are so-named because all the meteors appear to originate from northern Orion. But they’re also “Halleyids”, crumbs dropped by Halley’s Comet during its 76-year orbit of the sun.

Meteors appear to radiate from a point in the distance the same way snow striking your car windshield does when driving. Even though snowflakes and meteors are nearly parallel to each other, our eyes see them as converging in the distance. A similar illusion happens when looking down a set of train rails. Photo: Bob King

Each streak of light you see signals the incineration of a flake of Halley’s Comet, the parent comet of the Orionids. Every year in late October, Earth cuts across Halley’s orbit and bits of dust shed by the comet from previous passes near the sun burn up as they strike the upper atmosphere at speeds of 148,000 mph.

Few showers offer up faster meteors. Don’t bother pointing one out to an observing companion – it’ll be gone as soon as you open your mouth. Most tear across the sky in second or less.

We cross Halley’s orbital path twice a year, and each time we do, our planet slams into sand-sized bits of debris strewn by the comet during the many times it’s circled the sun. Our other encounter with Halley leftovers happens every around April 21 during the Delta Aquarid shower.

Watch for Orionids between about 2 a.m. and dawn. South or southeast is a good direction to face once you’re cozy under a big blanket to stay warm. The shower lasts a few days, so if the weather looks bad, try the mornings before and after Monday’s peak.

Perseid meteor shower puts on a great performance

A Perseid meteor shares the scene with the Pleiades star cluster (right) around 1:45 a.m. this morning Aug. 13 from Duluth, Minn. Credit: Bob King

I know many of you had clear skies and great meteor shower viewing the past few nights. Late last night my daughter Maria and I set up lawn chairs in the driveway, got under our blankets and watched the show together. With temperatures in the low 40s it felt more like fall than summer.

Cold often means VERY clear and the sky was spectacular. During an hour and a half we counted 60 Perseids and five non-Perseid meteors. We saw single faint meteors, a few super-nice fireballs that left bright trains and bursts of 2 or 3 nearly simultaneous meteors separated by lulls in activity lasting from 2-5 minutes.

It was really a lot of fun and a very relaxing way to spend time with my daughter. You’ve heard me say it before, but I highly recommend watching meteor showers both for the show and benefit of quiet conversation.

The graph below shows the ZHR (Zenithal Hourly Rate), which is the number of meteors an observer would see under a very dark sky with the radiant of the shower in zenith. Credit: International Meteor Organization

If lousy weather frustrated your attempts to see the meteor shower, the Perseids will continue to be active, although at much lower rates, through August 24. The International Meteor Organization has published preliminary results for the shower. A peak of 130 meteors per hour were recorded around 18:00 Greenwich Time (afternoon in the Americas) on Aug. 12.  Complete results HERE. Lots of photos HERE.

Gamma Delphinid meteor shower a no-show

Just a quick update. So far there have been no reports of a meteor shower from the constellation Delphinus earlier this morning. We appear to have missed the stream of debris left behind by the long-ago comet. Spaceweather’s Dr.Tony Phillips suggests it’s also possible the meteoroids were too small to produce meteors bright enough to see with the naked eye. I’ll have more information at a later time.

For those who went out for a look, I hope you enjoyed your time under the stars all the same.

Rare Delphinid meteor shower may erupt tomorrow morning

The Gamma Delphinids meteor shower – if it shows – will appear to radiate from the constellation Delphinus (del-FINE-us) the Dolphin high in the southern sky shortly before dawn tomorrow morning June 11. This map shows the sky facing south at 3:30 a.m. local time. Delphinus is to the east or left of the bottom of the bright 3-star figure the Summer Triangle. Created with Stellarium

June 11, 1930. Three meteor watchers in Maryland were quietly watching the sky when out of nowhere a half-hour-long bright outburst of meteors flared from the little constellation Delphinus the Dolphin. Since that time, observers keen on meteors have watched for an encore without success.

But there might be good news for this all-but-unknown shower. Peter Jenniskins, research scientist with the SETI Institute and NASA Ames Research Center, suggests that the cometary debris responsible for the Gamma Delphinids may return again to ignite a similar outburst tomorrow morning June 11.

The expected time of maximum activity is 4:30 a.m. Eastern Daylight Time, 3:30 a.m. Central, 2:30 a.m. Mountain and 1:30 a.m. Pacific and 11:30 p.m. (tonight) Hawaiian. No moon will spoil the view and Delphinus will be high in the southern sky as seen from the Americas. You’ll know you’re seeing a shower meteor if you can trace its path back to Delphinus, a small pattern of stars in the shape of a dolphin near the bottom of the bright Summer Triangle asterism.

The map above shows where the shower will be best visible (bright green-yellow band). Unshaded areas on the map won’t have a view of the shower. The higher the radiant (located in Delphinus), the more potential meteors might be seen. Credit: Geert Barentsen, International Meteor Organization

This is an oddball shower for sure, since no one knows how many meteors might be visible or even its duration – estimates range from one hour to 15 minutes with meteors appearing a minute or two apart.

If you’re in a gambling mood, unfold your reclining lounger and face south an hour before the expected maximum and keep watch. Even if the Gamma Delphinids fail to show you’ll have a fine view of the summer Milky Way. Bring binoculars and enjoy the rich star fields along its length. You’re also guaranteed to see at least a few random or sporadic meteors – typically 5-10 per hour.

Tonight June 10-11 from 10 p.m. – 2 a.m. CDT, Dr. Bill Cooke of NASA’s Meteoroid Environment Office will take your questions about the Gamma Delphinids via live web chat. He’ll offer viewing tips about the shower and include a live Ustream telescope view of the skies over Huntsville, Ala.

If you spot any Gamma Dels, please send a report to: lunro.imo.usa@cox.net Have video or images? Consider sharing them with the Office’s Flickr group. Any observations could help scientists unravel the shower’s origins and assist in predicting future outbursts.

And don’t forget, tonight (June 10) the newly-hatched crescent moon joins Mercury and Venus at dusk in the northwestern sky.

Close but still so far away – the sun at perihelion

Quadrantid meteor shower Jan. 2-3, 2013

Were you like me and got up this morning only to find the sky still overcast? No meteors for this poor astronomer. I hope some readers fared better.

It always seems to be clear over John Chumack’s home in Dayton, Ohio. Chumack, a long-time amateur astronomer, recorded 52 Quadrantid shower meteors overnight using a low-light video camera. Click the video above to watch it all go by in just 33 seconds.

Earth’s oval or elliptical orbit causes our distance from the sun and orbital speed to vary during a year. This week we’re both closest and moving fastest. Illustration: Bob King

At 10:37 p.m. January 1 this week, Earth passed an annual milestone in its orbit, reaching its closest point to the sun for the year. Astronomers call it perihelion, a Greek-rooted word combining ‘peri’ (close) and ‘helios’ (sun). Earth’s distance from the sun varies over the course of a year because our orbit is not a circle with the sun at the center. Rather it’s an ellipse – like all the other planets’ orbits – with the sun slightly off to one side.

On July 5 this year, Earth will reach its farthest distance from the sun called aphelion (AP-hee-lee-on). The difference from one side of our orbit to the other is only about 3 million miles or 3.3%. While the change in distance affects the amount of heat we receive from the sun, it’s not nearly enough to affect the seasons, which are caused by the 23.5 tilt of our planet’s axis. The tilt of the north pole toward the Sun in June causes summer north of the equator, while summer south of the equator comes six months later when the south pole is facing the Sun.

Difference in the size of the sun when Earth was at aphelion (top) last July and this week at perihelion. It’s very obvious in side-by-side photos but extremely difficult to discern with the naked eye. The difference amounts to just 1.1 minute of arc or 1/30 the diameter of the full moon. Credit: Giorgio Rizzarelli

Because our distance from the sun varies, so does the sun’s size and our planet’s orbital speed. When closest to the sun, Earth moves faster than when farther away, the same way sun-hugging Mercury orbits faster than distant Jupiter. Our average speed is 18.5 miles per second (66,600 mph) through space, but today we’re zipping along 2,160 mph faster than we will come July. I can almost feel the wind in my thinning hair.

The sun is peppered with sunspots in this photo made at  9 a.m. CST today Jan. 3 by the Solar Dynamics Observatory. Sunlight takes 8.2 minutes to arrive at Earth at perihelion and 8.5 minutes at aphelion. Credit: NASA

Giorgio Rizzarelli, a regular reader and commenter on this blog, performed an interesting experiment comparing the size of the sun at aphelion on July 5, 2012 and at perihelion earlier this week. The difference is immediately obvious from his unique perspective.

Giorgio went a step further and measured the difference in diameters to arrive at the Earth’s orbital eccentricity.

Eccentricity or ‘e’ refers to how stretched out a planet’s orbit is compared to a perfect circle. With a circle defined as e = 0, Rizzarelli calculated an ‘e’ of  0.017 (nearly circular but not quite) for Earth’s orbit, in excellent agreement with the published figure of 0.0167. (see calculation below). Amazing what you can do with a camera from your own backyard.

“The disc in lower photo is 3.4% bigger than in the upper, so (dividing by 2) 1.7% bigger than average. Hence Earth today is 1.7% closer to the Sun than average. This defines the approximately eccentricity, (which is) 1.7% or 0.017.” – Giorgio Rizzarelli