The entire universe in blog form

Dec. 21 2014 7:00 AM

Today the Sun Stands Still

Today is the real reason for the season: It’s the winter solstice! If you’re a purist, then raise your glass at 23:03 UTC (18:03 Eastern US time), because that's the moment the solstice occurs.

There are a lot of ways to look at this, but they all boil down to the Earth’s axis being tilted with respect to its orbit. You’ve seen this with classroom globes; they’re tipped by about 23.5°. As it orbits the Sun, the north pole of the Earth’s axis is always pointed pretty close to Polaris in the sky, which means that sometimes the axis is tipped toward the Sun, sometimes away. When it’s tipped as far from the Sun as it can be, that’s the moment of the winter solstice.


Illustration by Tfr000 on wikipedia, Creative Commons License. In this animation, the winter solstice occurs when the Earth is to the far left part of its orbit, and the northern axis points away from the Sun.

Of course, when the northern pole is tipped away the Sun, the southern pole is tipped toward it, so it’s summer down there. In that sense, it’s better to call today the “December solstice” rather than “winter solstice”. Nearly 900 million people live south of the Equator, so it’s probably a good idea to keep them in mind when we name things.

But for us in the north, today is the day the Sun stands still (the literal meaning of “solstice”). What does that mean? If you go outside every day at local midday (literally, halfway between sunrise and sunset) and note the position of the Sun in the sky, it changes during the year. It’s lower in the winter and higher in the summer.

Sun's path
The Sun's path across the stars during the year, seen at midday.

Illustration by Phil Plait

Today is the day it gets as low as it can at midday—that’s why it “stands still”; it’s dipped as low as it can go and has stopped its decline. It’s the shortest day and longest night of the year. If you go out tomorrow it will be a wee bit higher at midday, and the day will be a tad longer.

The change is slow at first, then speeds up, accelerating the most at the vernal equinox in March. On that day, the days are lengthening as quickly as they can, usually by a couple of minutes or so per day. Then, at the June solstice, the Sun is as high in the sky as it can get, days are at their maximum length, and the Sun stands still once again. It reverses course, and starts getting lower every day at midday until late December.

Lather, rinse, repeat. Unless you're in the southern hemisphere, where this is all upside-down, so for you austral folks: repeat, rinse, lather. 

And this is why we have seasons: In the summer the Sun gets higher in the sky, heating us more efficiently, and the day is longer, so there's more time to warm up. In the winter it's lower, and the days are shorter, so it gets cold. 

Some people call today the first day of winter, but I prefer to think of it as mid-winter’s day. After all, today the Sun starts getting higher in the sky, so why say that’s the first day of winter? Weather is regional, anyway, so trying to tag a definition of when winter starts is pretty silly.

Instead, use today to think about astronomy, cycles, the motion of the Earth, the patterns of the sky, and the amazing nature of our Universe.

Or, honestly, why not every day?

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Dec. 20 2014 7:30 AM

Detecting an Exoplanet… Without a Telescope

Years ago, when the first transiting exoplanet (HD 209458b) was found, I was startled to realize that it could be easily detected using a small, inexpensive telescope.

Transiting exoplanets are planets that orbit other stars, and from Earth we just so happen to see their orbit edge-on. That means it passes in front of their parent star (that’s the transit bit), blocking a fraction of its light. A tiny fraction, usually far less than 1%. But if the star is bright this dip in brightness can be spotted in small telescopes. I remember doing the calculations and finding that a 30 cm telescope could detect HD 209458b in a single night’s observations. Tough, but possible.


That meant an amateur astronomer could detect exoplanets! What didn’t occur to me at the time is that you don’t necessarily need a telescope to do so.

David Schneider, an editor at IEEE Spectrum, has described a setup using a digital camera and 300mm telephoto lens that has allowed him to detect the transit of the exoplanet HD 189733b, a so-called hot Jupiter, a massive planet orbiting very close to its star. The transit depth is about 2.6%, and his data look pretty good to me. He based his work on an amateur astronomer (vmsguy on the Cloudy Nights forum) who has also posted data that look pretty convincing.

Basically, the idea is to take several exposures over the course of the transit, taking care to make sure you get pictures taken before and after the transit. That’s your baseline. Using software to align the images and examine the stars (both vmsguy and Schneider used IRIS, which is Windows only, but other packages exist), you measure the brightness of the star over time to see the transit.

Not that it’s that easy! In reality you do relative photometry: You measure the brightness of many stars at the same time, so that a passing cloud doesn’t dim your star and make you think you’ve found an exoplanet. You also have to take other calibrations (like darks and flats), and apply them carefully. But it’s not impossible, and in fact sounds like fun.

Mind you, Schneider went all-in, even to the point of building his own gear to track the stars; but if you have a telescope you can always just use the motor drive that does that for you. The point is, you can detect exoplanets using just a camera, a good long lens, and a solid mount!

That’s amazing. I’ve been thinking of trying this sometime using my own 20 cm ‘scope; a lot of exoplanets are within range. But I’m still figuring out how to take astrophotographs, and believe me, I know how addicting this can be. I used to do this for a living, and if I get the software and start observing, I’ll be down the rabbit hole pretty quickly!

But in some ways, that’s the point. If you have the time and resources, it’s pretty amazing what you can do. You can even observe alien worlds.

Tip o’ the lens cap to James Walker.

Dec. 19 2014 2:15 PM

The Horsehead and the Flame

Well folks, looks like I have your next desktop wallpaper for you: a Spitzer Space Telescope image of the incredible Flame Nebula, a star-forming gas cloud hanging off Orion’s belt:

Dec. 19 2014 7:15 AM

Science, Celebrities, and the Perils of Promotion

Promoting science can be tricky. In general it’s fun and rewarding. I have a passion for science, and I wear it happily for all to see.

But there are minefields afoot. Of course there are people who deny science, and they will let the vitriol flow if you happen to stick a toe into their territory. There’s also the issue of diversity, including topics like women in science as well as people of different backgrounds, color, beliefs, and so on. I’m all for promoting more inclusion in science: the more the merrier! Reality is, and should be, for everyone.


But how to do this, how to actually promote these ideas, can get interesting.

A while back I was skimming my Twitter stream, and saw that my friend Christina Ochoa had retweeted a fun graphic created by Elise Andrew of I F’ing Love Science:

scientists actresses
Actresses, scientists.

Graphic by Elise Andrew

The picture is titled “Actresses with a passion for science” and shows five such women: Hedy Lamarr, Lisa Kudrow, Mayim Bialik, Natalie Portman, and Danica McKellar. I know how important it is to have good role models for kids and how girls need more support in getting into STEM (science, technology, engineering, and math) fields. Like it or not, actors and other famous people bear weight, so showing famous actresses who love STEM in my opinion is a pretty good thing.

So I retweeted the picture, adding “Love this” to it.

Then things got interesting.

Within minutes I started seeing responses about Dr. Bialik. Yes, “doctor”; she has a Ph.D. in neuroscience. The thing is, she also holds a number of beliefs with which I and many others disagree, some of them very strongly. For example, she’s a spokeswoman for a group called Holistic Moms—they support homeopathy, a provably worthless and arguably dangerous bit of “alternative medicine.” They are also strongly anti-vaccination, and Bialik herself supports anti-vaxxers (she has stated she has not vaccinated her own children, a position I am strongly opposed to).

I knew all this when I retweeted the picture. I’ll admit, I hesitated before doing so, specifically because of this. Is promoting this picture also promoting anti-science beliefs? Looking at the responses on Twitter, a lot of people think so. I see their point, but I also don’t think this is quite so black-and-white.

I do strongly disagree with many of Bialik’s beliefs. But I also know that she is a high-profile actress, starring in The Big Bang Theory where she plays a scientist. Her character, Amy Farrah Fowler, is a biologist and is commonly seen doing work in the lab and talking about her research with her friends. I’m quite fond of her character; she’s a passionate scientist, a decent person, a dork, emotional, analytical, and has trouble being objective when assessing her relationship with her significant other. I know a lot of people like that. I am people like that.

On top of that, Bialik has promoted science herself. For example, I urge you to take a few minutes and watch this delightful video where she does a great job talking up science:

Clearly, she can be a positive role model for science. However, we must have a care. The same people who might be inspired by her pro-science message might look into her more and find that she holds some less-supported beliefs, some that are anti-science.

So is using her in that montage of pictures a good thing or a bad thing? I would argue it’s neither, but the good outweighs the bad. The facts are that she is a scientist, she is an actress, and the picture was about actresses who are scientists. In point of fact, celebrities can be influential, and it’s a good thing that people see science supported by celebrity.

But of course we should also be careful not to put celebrities on too high a pedestal. Yes, Bialik has beliefs unsupported by science. But so does everyone. I imagine if we dig into the histories of the other four women shown in the picture we’ll find all sorts of things that go against the foundations of science, just as you would if you examined anybody’s thoughts. I have met my fair share of scientists who believe in one thing or another without evidence, or despite it. Heck, you can find Nobel scientists who fall into that category, ones who have supported clear crackpottery.

I’ll note I’ve dealt with this before when I was at an event with a Miss Utah; though she and I would disagree strongly on a number of topics, she was also an outspoken promoter of STEM, which is why she was there. And I was glad she was there, doing what she did. Think of it this way: If you knew of someone who did a great job taking down psychics, but also thought global warming was a hoax, would you then stop praising them for their work against psychics? It’s not an either-or thing; I would hope you would continue to praise them where appropriate but also take them to task where needed, too.

While you might dismiss those ideas and think less of the person holding them, that doesn’t necessarily subtract from their contributions to science. In this case, Bialik has done a lot to raise awareness of science and women’s contributions to it. Celebrating her (and the other four actresses) for that is great, and that was the sole purpose of the picture, and it’s appropriate to praise her there.

That doesn’t mean I am forgiving of Bialik’s beliefs at all. And in fact her presence in the picture has brought attention to them, which I think is also a good thing. A lot of folks agree with her when it comes to health issues, and that’s a big problem in this country; I’ve been clear about that for years.

That’s what I meant about this not being black-and-white. We’re all shades of gray, and if you really only want to praise people who are absolutely the perfect icons of science in every way, well, good luck finding them. You’ll be looking a long time.

As for me, I will continue to support science the best I can, and also support women in science. That’s the bigger picture here, and one we should all bear in mind.

Note: The friend I mentioned above, Christina Ochoa, is part of a group of actresses (also friends of mine) who love science. They call themselves Scirens and you should follow them on Twitter. They're good people, and I’m pleased to help support their efforts.

Tip o’ the calculator to Joel Parker for the link to the video.


Dec. 18 2014 11:30 AM


I love photos of the Earth taken from space; our deserts, oceans, islands, volcanoes, farmland, forests …  all of it.

But there’s something special about seeing something recognizable, even iconic, from space. Perhaps we’re used to seeing such things on maps, but a photo of it adds the dimension of reality.


I’m not sure. But no matter why, it’s hard to deny this is just straight-up cool:

I’ve spent a lot of time on this peninsula; family vacations when I was younger, visiting friends when I was older, watching the odd rocket launch or three. My folks lived there for many years, so seeing this from space reminds me of combing beaches for shark teeth when my daughter was little, getting sunburned like an idiot despite slathering on lotion, sweating maniacally in March.

At night, from space, the outline of Florida makes it so obvious (like Italy; perhaps peninsulae are easier to recognize). The lights of the city are both lovely to see and appalling to seriously consider; the light pollution is overwhelming, ironically drowning out everything in the night sky except for the few brightest objects … like the International Space Station passing overhead, from where this photo was taken.

Our technology has made it possible to go up and look down, but much harder to stay down and look up. If there is some sort of allegorical conclusion to be drawn here, well, I’ll leave it for you to consider.

Dec. 18 2014 7:15 AM

Scientists Discover 38 Percent of the Earth

OK, so the title is a little tongue-in-cheek, but it's sorta true: Mineralogists have finally found naturally occurring samples of what may be the most common mineral on Earth: what’s called silicate perovskite, or (Mg,Fe)SiO­3.

They’ve also officially given it a name now too: bridgmanite. Percy Williams Bridgman won the Nobel in 1946 for studying high-pressure minerals … and that’s a clue to why this mineral was so hard to identify.*


Bridgmanite can only exist under conditions of high temperatures (at least 2,100 C) and pressure (240,000 times the sea level atmospheric pressure—a crushing 240 metric tons per square centimeter!). It’s thought to be abundant in the Earth’s lower mantle—a region 660 to 2,900 kilometers beneath Earth’s surface. The molten rock in the mantle is fluid, moving incredibly slowly inside our planet. Any bridgmanite in the mantle brought up toward the surface slowly breaks down under the cooler and lower pressure conditions, which is why it’s remained elusive, even though the mineral may make up as much as 90 percent of that part of the mantle (and therefore more than a third of the entire planet).

Layers in the Earth. 1) Inner core, 2) outer core, 3) lower mantle (the layer in question), 4) upper mantle, 5) lithosphere, 6) crust.

Drawing by Mats Halldin

The scientific break came in the form of a meteorite, called Tenham. Long ago, two asteroids collided, and the impact created high temperatures and pressures. Bridgmanite formed, and the piece cooled too rapidly for the mineral to decompose. In 1879 the rock fell to Earth in Australia, where it was found and eventually determined to have different kinds of high-pressure minerals in it. Bridgmanite exists in it in very small grains, typically only about 1 micron wide (a human hair is typically 100 microns in width), but it’s there. It was announced earlier this year, but the scientists just published their paper about it in November.

This is quite a boon! It’s difficult to reproduce the conditions in the deep Earth, and even if you can it’s even harder to study what you get. In this case, it’s like we got a sample of the Earth’s lower mantle for free. It’s also a nifty crossover between different disciplines: meteoritics, high-pressure physics, mineralogy, just to name some.

And also, it’s just amazing. We live on a ball of rock and metal 12,740 km across, with a staggering 1 trillion cubic kilometers of material in it, the vast vast majority of which we can never directly see. I wasn’t even aware that we didn’t actually know for sure what made up over a third of our own planet.

Science! Astronomy may be my passion and my love, but sometimes it’s good to remember that science also tells you, literally, what’s going on right underneath your feet.

Correction, Dec. 18, 2014: This post originally misstated when Bridgman won the Nobel. It was in 1946, not 1964.

Dec. 17 2014 11:15 AM

Dark Sky in Canarias

Because why not, here’s a luscious time-lapse animation of the sky over La Palma, Tenerife, and El Hierro, three of the Canary Islands off the coast of Morocco:

This video was taken by photographer Imanol Mujika. He has a stunning Flickr gallery, too.


I’ve been to La Palma, and the clouds really do roll in like that. I like how you can see them swell and disappear over the city (I think it’s Santa Cruz in the video) like waves on a beach.

Also, toward the end (at the 1:55 mark), there’s a star trails shot where the long exposure shows the stars as streaks due to Earth’s rotation. Stars on the celestial equator—the part of the sky directly above the Earth’s equator—make straight lines, but toward the right (north) and left (south) they curve more, as they circle the pole. But they curve in opposite directions!

That’s just the natural consequence of the wide-angle shot, being able to see the motions of stars across a big chunk of sky. Near the celestial poles, the stars make smaller circles, so we see the curvature of their trails changing with position. I have a more detailed explanation in an earlier post, if you’re curious (and you should be!).

Seeing this makes me want to get under the stars again ... and now that it's winter, Orion, Taurus, and all the wonderful chilly weather stars are back at a decent time of night. Time to warm up my camera ...

Dec. 17 2014 7:15 AM

Hour by Hour, the Phases of the Moon for the Entire Year of 2015

Just the other day I wrote about the good folks who create video at NASA’s Goddard Space Flight Center. And now I get to do it again: They just released their Dial-a-Moon page for 2015, which lets you display the hour-by-hour appearance of the moon for the entire year.

They also put out a video compiling all the images for the year into a single animation. You might expect it to look like the Moon is just sitting there, with the phase changing as the terminator (the day/night line) sweeps across its face. But that’s not what you get at all. Watch:



The Moon orbits the Earth in the same amount of time it takes to spin once. This means it always shows us the same face … except not really. The Moon’s rotation is constant, but the velocity it travels in its orbit around the Earth changes because that path is an ellipse. When it’s closer to Earth it moves faster, and slower when it’s farther away. This mismatch lets us peek a bit “around the side” of the Moon. The Moon’s spin axis is also tipped a bit with respect to its orbit, and that allows us to see over the northern and southern poles, too.

When added together, you get that mesmerizing nodding and weaving motion, which is called libration.

The video has some nifty extras too. At the top left it shows the Moon’s position in its orbit around the Earth, as well as the phases of both the Earth and Moon.

Behind the big Moon in the center is a line representing the Moon’s orbit seen edge-on. On the left is the Earth, and on the right it shows how far away the Moon is, in units of the Earth’s diameter (12,740 kilometers or 7,900 miles).

On the bottom left is a diagram of the Moon, with a blue and yellow dot; the blue is the sub-Earth point and the yellow is the sub-solar point.  In other words, if you were standing on the Moon at the position of the blue dot the Earth would be exactly overhead, and if you were at the yellow dot the Sun would be directly overhead. Note that when the Moon is full to us on Earth, the yellow dot is smack dab in the center near the blue dot: The Sun is shining straight down on the half of the Moon we see. When the Moon is new (completely dark), the yellow dot is on the far side of the Moon; the Sun lights up the half we can’t see, and the half facing us is dark.

Finally, on the lower right are lots of fun numbers: date/time, the phase (percentage of the Moon illuminated as seen from Earth), how big it appears, how far it is, and so on.

If you’re planning any detailed lunar observations, the GSFC page is pretty useful. I know I’ll use it to plan our future Science Getaways trips; we try to schedule them around new Moon so we can see the stars when I take my telescope out.

And even if you don’t need that kind of detail, this is just a way cool thing to have around. There’s also a video that shows just the Moon without any of the annotation, and a view as seen from the Earth’s Southern Hemisphere as well, with the Moon upside-down. Wacky Southern Hemisphereans.

And don’t forget this is a simulation, based on images taken by the Lunar Reconnaissance Orbiter. The real thing is even better, so go outside and look! You may get inspired. A couple of friends of mine were.

Dec. 16 2014 12:24 PM

Meteor Not?

So in my post about the Geminid meteor shower yesterday, I said that I didn't catch a single Geminid in my photos, and that's true. But going over them carefully, I happened to see something a bit weird, and I'm not sure what to make of it.

I took many shots of Orion, since it was perfectly placed over a tree, and any meteors going across it would make for a great photograph. I kept the exposures to 20-30 seconds, since the sky background was pretty bright, and I didn't want the stars to trail too much. While I didn't get any Geminids, I did happen to see this tiny streak:


You can see the three stars in Orion's belt at the top, and the fuzzy glow of the Orion Nebula, the nursery to a lot of young bright stars. And there, just below the third star in Orion's "dagger," there's that little blip. Is that a meteor?

I'm not sure. It's not a Geminid for sure; given Orion's position in the sky, a Geminid would leave a left-to-right streak in this photo. It's not in the previous or following photo, taken seconds earlier and later. 

I'm not sure what to make of it. Most meteors would leave much longer streaks, but if the bit of cosmic debris happened to be heading almost straight toward me, it would leave a short streak due to perspective. If it's not a meteor, what could it be? Sometimes subatomic particles can leave similar streaks in a digital detector (these are typically called cosmic rays), but I've never seen one in my usual use of a regular camera. It may have been a bird lit by city lights, but the streak doesn't wiggle, and is so short that seems pretty unlikely. Same for an insect much closer to the camera. 

It seems weird to think that a small bit of interplanetary debris sloughed off by some ancient collision between two asteroids millions of years ago and hundreds of millions of kilometers away may be the least unlikely explanation, but there you go. 

Astronomy does sometimes provide an unusual perspective.

Dec. 16 2014 7:30 AM

50 Shades of 67/P

That picture shown above is, seriously, a full-color photo of the comet 67P/Churyumov-Gerasimenko.

It was taken by the Rosetta spacecraft on Aug. 6, 2014, when the probe was still 120 kilometers (75 miles) from the comet (long before the Philae lander was deployed). The OSIRIS camera on board has red, green, and blue filters that allow the camera to mimic what the human eye sees. It’s not exact, but it’s close.


And what you see is … gray. Which means the comet really is just kinda overall gray.

That doesn’t surprise me. Comets aren’t really loaded with the sorts of colorful minerals that make Mars or Europa or even our own Earth so gloriously hued. They’re mostly water ice and rock, with other things thrown in for good measure.

But you might expect some variation when you look at the comet in detail. But it’s very smoothly gray; there’s very little change in the color composition across the comet. That means there’s probably physical homogeneity across its surface. If there are any interesting minerals or materials in the comet, they appear to be distributed pretty well.

That does surprise me; I was expecting to see patches of ice at least on the surface, and those reflect blue light better than red. But we see no blue patches at all. The water ice in the comet is mixed in with the other stuff.

That’s not the case with other comets; for example, Hartley 2 is also double-lobed, with a waist in between them, similar in shape to 67P. But observations using the EPOXI spacecraft show the waist is emitting water ice, while the lobes blow out more carbon dioxide. The waist is also smooth in appearance, while the lobes are rougher. It’s unclear why this might be.

But 67/P, for all its similarity in overall shape, is clearly a different beast than Hartley 2. That’s telling us something. Perhaps they were born in different parts of the solar system, and so are constructed differently (we know that to be the case for some by looking at isotope ratios in different comets). Maybe something happened as they aged—4.55 billion years is a long time, after all—that changed them. It could be that 67/P's outgassing and dust have coated its surface everywhere. Or maybe comets are just a diverse group, every one different from another. None of these circumstances would be surprising.

There’s another possibility, too: Simulations of the early solar system show that our Sun may have stolen the vast majority of its comets from other stars! If that’s the case, then that would go a long way toward explaining why comets are so different from each other. They were born in different solar systems!

It’s hard to express just how awe-inspiring that is. We’ve always assumed comets were like time capsules from the ancient solar system (if weathered and worn over the eons). But they actually may be samples of alien stars, transplants from elsewhere in the galaxy.

Thinking about this literally raises the hairs on the back of my neck.

So gaze upon that photo of 67/P once again, and think about what you may be seeing. I know I’ll never use the word “gray” to mean boring ever again.