Star Struck

Mars Opposition 2016

That increasingly bright and obviously red object rising in the southeast late at night (around 10:30 pm EDT from Lynchburg) is Mars. It will rise ever earlier as it moves into position exactly opposite the sun in our sky on May 22, when it will rise around 8:30 EDT. This every-26-month event is a Mars opposition, and this is a reasonably good one.

What makes one opposition “better” than another? Therein lies a tale of orbital peculiarities that allowed the true nature of our solar system to come to light.


The orbits of planets around the sun are not perfect circles, they are in fact ellipses, circles that have been pulled and stretched. The ellipse below varies from circularity far more than any planetary orbit in our solar system, but it illustrates the point.


Earth’s orbit is elliptical, but not very much so. We are roughly 3 million miles (5 million kilometers) closer to the sun in January than we are in July, with an average distance of 93 million miles (150 million kilometers). But Mars! Mars has the second most (after Mercury) elliptical orbit of the eight planets (sorry, Pluto lovers) and that means that not all Mars oppositions are created equal.

The wonderful diagram below shows the positions of both Earth and Mars for all oppositions between 2012 and 2027. The distances between the two planets are given in astronomical units (AU) where one AU is that average distance between Earth and the sun. Mars, further from the sun than the Earth and therefore moving more slowly around it, takes 687 days for one orbit. The oppositions will occur at different places around that orbit, and only when the faster-moving Earth has caught up to the more stately motion of its sister planet.


The opposition of 2027 is an example of a “bad” opposition. Mars is near its aphelion (farthest distance from the sun, marked by the orange A), and so the distance between the two planets is 0.6780 AU, or 63 million miles (101 million kilometers). By contrast, the 2018 opposition is a very “good” one. Mars and Earth line up almost exactly halfway between perihelion (closest distance to the sun) for Mars and aphelion for Earth. The planetary distance is 0.3862 AU: 36 million miles (58 million kilometers). Quite a difference!

The difference between these two oppositions is seen in the greater brightness of Mars in our skies with the nearer opposition, and the greater apparent size of its disc. In the days before we had robots roaming its surface, Mars was eagerly scanned with the most powerful telescopes of the day at each opposition, particularly at very favorable ones.

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The 29th of February

A relatively rare event—an extra day in February, the every-four-years February 29th—greets us again this Monday. Why does this happen? As you might expect from my posing the question on this blog, the answer is wrapped up in astronomy.

In fact, our whole calendrical system is based on astronomy. The year is based on the time required for Earth to complete one circuit of the sun. The month is (loosely in a solar calendar, exactly in a lunar calendar) the time between repeating lunar phases, known as the moon’s synodic period. A day is the time of one rotation of Earth on its spin axis. Even a week of seven days is based on the seven naked-eye objects known to the ancients as “planets”: the sun and the moon along with Mercury, Venus, Mars, Jupiter, and Saturn.

But these units of time don’t fit neatly into each other. There are about 12.4 synodic periods of the moon in a year, not exactly 12. We compensate for this with months that are mostly longer than the 29.5 days of one synodic period. And there are 365.2425 days in a year, not 365. Hence a periodic February 29th.

You’ll notice that the “extra” time in a year is pretty close to one fourth of a day. So every four years, we add an extra day to February.

But wait. It isn’t exactly one fourth—it’s a little less. After 400 years of adding a day every four years, we would have added a total of about three extra days; we have to compensate somehow. We do so by not adding February 29th in three out of four century years. We only have a leap day in century years that are exactly divisible by 400.

Here’s how it works. 1896 was a leap year, as was 1904. But 1900 was not. It is a “century year”, but it is not divisible by 400. The year 2000, however, was a leap year.


The effect of this on the time of the northern summer solstice—the exact moment when the sun reaches its northernmost point in the sky—can be seen in the graph. Notice how the trends for the 18th, 19th, and 20th centuries all moved a little lower, corresponding to later dates. The summer solstice reached its latest point in 1903, roughly 3 pm on June 22nd, Greenwich Mean Time (GMT). And note how this trend would have continued if the year 2000 had not been a leap year. 2003’s summer solstice would have been even later than 1903’s; instead, it was around 8 pm on June 21st.

As we moved into the second half of the 20th century, the precision of our clocks improved to the point that the motions of the Earth, its revolution around the sun and its rotation on its axis, were shown to be too variable. Time itself was redefined in terms of the frequency of a particular atomic energy transition. For those who care to know, the official definition of a second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. One minute is 60 times this, and one hour is of course 60 minutes.

Periodically, a leap second is added to keep atomic clock time in sync with what is known as mean solar time. 26 such leap seconds have been added since this began in 1972, the last coming on June 30, 2015.

Do we ever subtract a leap second? No, the extra time is necessary because the rotation of the Earth is very slowly but inexorably decreasing. Each day is ever so slightly longer than the day before.

So use the extra time, whether an extra day or an extra second, to good advantage! I plan to have a nice lunch with a new friend on this particular February 29th.


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That Einstein Was a Smart Guy

A long time ago in a galaxy far, far away…

Two black holes collided and merged, releasing unimaginable quantities of energy in the form of gravitational waves. Last year these waves were detected on Earth by an exquisitely sensitive pair of instruments near Hanford, Washington and Livingston, Louisiana, and last week that detection was announced to the world. Unless you have given up all media for Lent, you have surely heard of it by now.

So what’s the big deal? Why are scientists so excited? Why is there talk of a Nobel Prize? What are gravitational waves, anyway?

Einstein and Gravity

We’ll start the story at a time much more recent than the ancient event whose signals only reached Earth after more than a billion years of travel: almost exactly a century ago, in fact. The name of Albert Einstein was known to the world of physicists, but had not yet become synonymous with scientific genius to the public at large. After publishing his Special Theory of Relativity in 1905—this is the one that predicts clocks running at different rates as they approach the speed of light, along with other non-intuitive results—Einstein worked for years to generalize his theory. The 1905 work was “special” because it didn’t really incorporate the effects of gravity into its equations. This is not much of a problem except in very strong gravitational fields.

The General Theory of Relativity published in 1915 took full account of the effects of gravity. The Special Theory had already shown that space and time could no longer be considered as independent of each other, that they merged into a single entity best described as four-dimensional space-time.

I’ve never met anyone who could visualize four dimensions. To help us understand what is going on, it’s helpful to drop back to our familiar three dimensions, and visualize a stretchable rubber sheet. For the sake of convenience, we’ll put a grid on it. You’ll see why in a minute.

Here you go. Kind of boring, isn’t it? This is “flat” space-time, with no gravitational effects to be seen. Light traveling from one side to the other will follow a straight path along one of the grid lines. And what causes gravitational effects? Well, matter will do it—the more the better. And what matter does to space-time is to change its shape. In the presence of matter, space-time is curved.

The greater the mass, the greater the curvature of space-time. Einstein had described gravity in an entirely new way. Instead of its being some mysterious force that magically reached out from the sun to influence the planets, it was simply our description of how space-time was curved by the massive sun, and how the planets naturally responded to that curved space-time. In the classic words of John Wheeler: “Mass tells space-time how to curve, and space-time tells mass how to move.”

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Pluto Is Still Not a Planet

Sorry, Pluto lovers. A recently published paper by Jean-Luc Margot of UCLA (as a Star Trek Next Generation fan, I had to give the author’s full name) proposes a mathematically rigorous way to define a planet. Pluto, for all its undeniably fascinating appeal—it just doesn’t make the cut.

The official body tasked with naming and defining astronomical objects is the International Astronomical Union (IAU), of which most people had never heard until 2006. That was when the IAU gave official sanction to what astronomers had known for years, that Pluto was qualitatively distinct from what we now think of as “classical” planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The decision prompted millions of people to mostly good-natured outrage. You mean my fourth-grade teacher lied to me? Why can’t those scientists get their story straight? It didn’t help that the proposed definition was both vague and confusing. Here is the original IAU definition.

“A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”

Well, that first criterion is simple enough and explains why the moon (which orbits Earth) is not considered a planet. It does exclude any possible planets that orbit other stars, but we’ll get back to that.

The second criterion is hard to judge. How “nearly” round does the planet have to be? No one would argue against calling Jupiter a planet, but its fluid composition and rapid rotation flatten it at the poles and bulge it at the equator. It is certainly not perfectly round! The dashed line in the image below is a perfect circle.

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What’s Up?

Two short-lived sky phenomena are on tap in the near future, one taking place only tens of miles above us, the other millions of miles away.

Geminid meteor shower:

Each December we are treated to one of the better meteor showers of the year. The Geminids (so named because they appear to originate from the constellation of Gemini) promise rates as high as one meteor per minute at their peak. They are well situated for early evening viewing, and they come at a time of year when sunset comes very early. Rising at 3 am is not necessary! And this year there will be no moon in the sky to spoil the view. The peak of the shower will be the evening of Sunday, December 13, but there are meteors visible for a week before and after this date.
A few basic reminders are in order. Although meteors belonging to a single shower appear to originate from a specific point called the radiant, you can see them all over the sky. Their apparent paths are a matter of perspective; their paths appear to converge for the same reason parallel railroad tracks appear to converge in the distance.


Although the stuff of meteors (bits of cometary dust and particles generally no bigger than a grain of sand) comes from far away, the incandescent streak that marks their demise is generally between 50 and 75 miles above us. Find a site as far away from artificial lights as you can, lie on a blanket or a reclining lawn chair, and look up. No need for binoculars or a telescope–they are useless for viewing meteors. If you want to look in any particular direction, the radiant will rise above the northeast horizon around 7 pm and gradually move across the southern sky. It reaches its highest point around 2 am, but meteors will be visible all night long.

Happy viewing!

Comet Catalina:

Comets come in two somewhat arbitrary categories: short period ones that complete a single orbit of the sun in less than 200 years, and long period ones that take, well, longer. Comet Catalina falls into the latter category. It comes from a vast region of icy objects far from the sun called the Oort Cloud. Inferred by tracing back the paths of these first-time visitors to the inner solar system but never actually directly observed, the Oort Cloud surrounds the sun in a spherical distribution, with its members not confined to the flat plane occupied by the planets.

Oort-cloud diagram

Occasionally some gravitational perturbation will start one of these iceballs on a long, slow drop into the sun’s gravity well. With orbital periods in the tens of thousands of years, these are one-time visitors for all practical human purposes. They are pristine in the sense that the volatile materials that are frozen solid in the Oort Cloud have never—or at least seldom–been vaporized by a close passage to the sun, and represent well the primordial composition of these 4.5 billion year old relics.

Comet Catalina (its formal designation is C/2013 US10) is typical of a long period comet in that its orbit is highly inclined to the ecliptic plane in which planets orbit, and retrograde—clockwise from above the north pole as opposed to the counter clockwise orbits of all the planets.


Catalina will require binoculars to see it. Right now it is in the early morning sky close to Venus (the brightest object in the east before sunrise). Here is a finder chart for you.


And here is the comet in all its glory.


The ion tail (blue) points directly away from the sun; the dust tail (yellow) trails behind as its heavier and slower-moving components are pushed away from the comet’s nucleus by the solar wind.

Get those binoculars out!

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Planet Lineup

These beautifully clear fall mornings we’ve been experiencing have offered a rare opportunity to see four of the five naked-eye planets all lined up for our viewing pleasure. This morning at 6:45 am EDT, this was the view on Lynchburg’s eastern horizon. The almost vertical blue line is the ecliptic, the plane of the Earth’s orbit around the sun. Since all of the planets orbit the sun in very nearly the same plane, all of them will appear near this line in the sky.

October 18 2015

So where is Saturn, the only one missing? Currently it is on the other side of the sun as we view it, rising about 10:30 this morning, invisible without a telescope in the daytime sky, and best viewed shortly after sunset in the western sky.

The ancients spoke of seven planets: the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. Today we don’t consider the sun and the moon to be in the same category as the other five, but the word planet actually means wanderer. These seven objects did not stay put in the sky! Unlike the well-behaved fixed stars, these celestial objects moved across that stellar background. We know now that this is because they are so much closer to us than the stars. The stars do in fact move, but their great distance makes that motion difficult to detect over a human lifetime.

Have you ever wondered why we have seven days in a week–why this arbitrary number and not some other? Seven planets, seven days. Some of our day names reveal their origin in English: Sunday, Monday, Saturday. Others are more apparent in other languages such as French: Mardi (Tuesday), Mercredi (Wednesday), Jeudi (Thursday), and Vendredi (Friday). We can all be grateful that Uranus was not discovered until the era of the telescope, thereby sparing us from decades of middle school jokes.

Posted in Sky Phenomena, Solar System

Space Station Pass in Lynchburg Area

It looks as though Lynchburg area sky watchers will actually have clear skies for a celestial event! The International Space Station will be visible tonight in an especially bright and high-in-the-sky pass. It will appear low in the southwest at 7:13:33 pm EDT, reach its highest altitude of 72° in the northwest at 7:16:47, and disappear above the northeast horizon at 7:20:02. For those of us who still remember a time when the only satellite of Earth was the moon, this is an event we never take for granted.

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Total Lunar Eclipse

It seems as though Lynchburg’s record of clouding over for interesting celestial events is going to hold true for this weekend’s total lunar eclipse. But just in case all the forecasters are wrong, and for those of you who live where clear skies reign, here is the relevant information.

First, just the basic information concerning timing, then more details for those of us who like that sort of thing. You may have seen information giving the date of the eclipse as September 28, but for observers in North America it will occur late in the evening of Sunday, September 27. All times given are EDT.

  • Moon enters Earth’s umbral shadow; you will begin to see a dark shadow creeping across the moons face: 9:07 pm
  • Totality begins; the moon is fully within Earth’s umbral shadow: 10:11 pm
  • End of totality; moon begins to exit umbral shadow: 11:23 pm
  • Moon is completely out of umbral shadow: 12:27 am on September 28.

Or more succinctly, the total eclipse lasts from 10:11 pm EDT to 11:23 EDT on Sunday night, September 27.

So what’s up with this umbral shadow thing? Remember first of all how a lunar eclipse occurs: Earth comes between the sun and the moon and casts its shadow on the moon.

03_Lunar Eclipse

Now imagine yourself standing on the moon. If the moon and you are within the umbra, Earth completely blocks the sun. Within the penumbra however, only some of the sun’s disk is obscured by Earth. So while the Sun’s light is completely obscured within the umbra, it is only partially obscured within the penumbra. Totality occurs only when the moon is completely within the umbral shadow.

So the moon ought to be completely dark during totality, right? As anyone who has seen a lunar eclipse can tell you, it isn’t. This is more like it.

If the earth were an 8000-mile diameter ball with no atmosphere, the moon would indeed be dark. But our atmosphere bends (or refracts) the sunlight passing through it, and the light that is bent least is long wavelength red or orange light.

The darkest eclipses occur when the moon passes through the exact center of Earth’s umbral shadow, which is seldom the case. For this eclipse, it passes nearer the edge.

September 27 lunar eclipse

And the parts of the Earth’s surface where it will be visible?

September 27 lunar eclipse visiblity

The next total lunar eclipse with this optimal visibility for Americans will not be until 2019. But well before then, we will have a total solar eclipse in August 2017 that will reach its peak in mid-America where Missouri, Kentucky, and Tennessee come together.

You will not want to miss this—experiencing a total solar eclipse is something that should be on everyone’s bucket list. Witness the excitement in this video taken aboard the cruise ship where my wife and I witnessed the solar eclipse of July 2009. It is an amazing natural phenomenon!  And yes, I was whooping and shouting along with everyone else.

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New Horizons Update

If you’ve been wondering why you haven’t seen any more images from the Pluto flyby lately, here is the reason:


But there is exciting news from the New Horizons team! They have selected a tentative target among Kuiper Belt Objects (KBOs, distant and icy solar system bodies of which Pluto is only one of the larger representatives) for a future flyby. The candidate for a close up look has the prosaic name of 2014 MU69, a name which reveals its order of discovery among similar objects. It’s enough to know that it was discovered only last year.
This KBO is especially intriguing because it is believed to have formed in situ, where it is orbiting now, meaning that it should be essentially unaltered since its creation 4.5 billion years ago.
It is also more easily accessible than other possibilities. The image below shows the relative positions of the outer planets (Jupiter, Saturn, Uranus, and Neptune), Pluto, and 2014 MU69, both on the date of the Pluto flyby in July 2015 and on the date of the anticipated 2014 MU69 flyby in January 2019.

New Horizons

You may notice that in the 3 ½ year interval between these two flybys, only Jupiter and Saturn have noticeably changed their positions. Pluto and 2014 MU69 have barely moved at all! Objects orbiting this far from the sun move much more slowly than do, for example, the inner planets. While Earth’s average orbital speed around the sun is 30 kilometers per second (19 miles per second), that of Pluto is only 6.1 km/s (4.7 mi/s).
Keep in mind that this rendezvous date is only approximate. As the spacecraft draws nearer, its path may be refined as the target is better resolved, and the time may shift slightly. Indeed, one of the attractions of this target is that very little fuel expenditure should be required to come very near it. When you are more than 6 billion km (4 billion miles) and 6 hours in radio transmission time away from home, you have to conserve your resources!

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Over the next 16 months, the data collected during the New Horizons spacecraft’s mid-July flyby of Pluto will find its way back to Earth, a few bits at a time. Already there are surprises in the data so far received. How could there not be? We are seeing a whole new world revealed in detail for the first time in human history. The analyses and debates will surely only have begun once all the data are in.
The image below on the left is from the Hubble Space Telescope, and until July 2015 it was the best we had. The true-color image on the right is from the New Horizons flyby.
Pluto before and afterI thought it might be useful to write on what we have learned about the small icy worlds of the solar system from our previous explorations. Perhaps a more appropriately humble description would be that this is what we think we know! Each new world forces us to re-examine our assumptions, each new question begets ten more. Such is the nature of science, and the source of its endless fascination for so many of us.
Worlds of Rock, Gas, and Ice
Leaving out a lot of detail, we can think of solar system bodies as being composed of rock, gas, ice, or some combination thereof. The four innermost planets (Mercury, Venus, Earth, and Mars) are relatively small and rocky, and the four outermost (Jupiter, Saturn, Uranus, and Neptune) are mostly gaseous, with ice becoming more prominent as we reach the more distant environs of Uranus and Neptune.
800px-Size_planets_comparisonAmong the inner planets, Mars has two tiny rocky moons that may be captured asteroids, and the Earth has a rocky moon that is likely the result of a massive collision early in its history. Neither Mercury nor Venus has a moon.
The inner solar system has numerous smaller bodies collectively called asteroids that are mostly rocky. The greatest numbers of these lie between Mars and Jupiter, or co-orbiting ahead of and behind Jupiter in its orbit. Icy bodies wander into the inner solar system from farther out, spewing gas and dust tails as they are warmed by the sun. These are the comets that periodically grace our skies.

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