Solstice drift, and how to fix it

The summer and winter solstices happen around the 20th of June and December, respectively. Around the 20th? That seems rather… imprecise, for an astronomical event with a precise definition: the time at which the Sun reaches its “highest or lowest excursion relative to the celestial equator on the celestial sphere” or, for the viewer standing on the Earth, its highest or lowest altitude from the horizon. This is determined by the Earth’s orbit and corresponds to the time at which your current hemisphere’s pole points most closely to, or farthest from, the Sun. So why doesn’t it happen at the same time each year?

Inspired by an awesome book I recently acquired (“Engaging in Astronomical Inquiry” by Slater, Slater, and Lyons), I decided to investigate. I used the Heavens Above site to pull up historical data for the summer and winter solstices going back to 1980. I plotted the time for each solstice (in Pacific time) as its offset from some nearby day (June 20 or December 21). And sure enough, here’s what you get:

The solstice time gets later by about 6 hours each year, until a leap year, when it resets back by 24 – 6 = 18 hours.

Of course, the solstice isn’t really changing. The apparent change is caused by the mismatch between our calendar, which is counted in days (rotations of the Earth), and our orbit, which is counted in revolutions around the Sun. If each rotation took 1/365th of a revolution, we’d be fine, and no leap years would be needed. But since we’re actually about 6 hours short, every 4 years we need to catch up by a full rotation (day).

Now, we all know about leap years and leap days. But this is the first time I’ve seen it exhibited in this way.

Further, you can also see a gradual downward trend, which is due to the fact that it isn’t *exactly* 6 hours off each year. It’s a little less than that: 5 hours, 48 minutes, and 46 seconds. So a full day’s correction every four years is a little too much. That’s why, typically, every 100 years we fail to add a leap day (e.g., 1700, 1800, 1900). 11.25 minutes per year * 100 years = 1125 minutes, and there are 1440 minutes in a day. But that’s not a perfect match either… which is why every 400 years, we DO have a leap day anyway, as we did in the year 2000.

This is what, in computer science, we call a hack.

And now it is evident why for every other planet, we measure local planet time in terms of solar longitude (or Ls). This is the fraction of the planet’s orbit around the Sun, and it varies from 0o to 360o. It’s not dependent on how quickly the planet rotates. It’s still useful to know how long a planet’s day is, but this way you don’t have to go through awkward gyrations if the year is not an integral multiple of the day.

By the way, you can get a free PDF version of ‘Engaging in Astronomical Inquiry’. If you try it out, I’d love to hear what you think!

Detecting exocomets around other stars

We’ve heard many discoveries over the past decade of planets around other stars. Today astronomers announced the detection of seven new comets around other stars. You can read more here: “Exocomets may be as common as exoplanets”.

Why should we care? Well, comets are a lot smaller than planets, so it’s impressive that they can be detected at all. Even more impressive, these discoveries were made with a 2.1-m telescope on the ground, not in space. You might be wondering how exactly they were detected if it’s so hard even to find Earth-sized planets. That’s because these comets aren’t being detected directly, e.g., by the brief drop in stellar brightness when a planet transits in front of it. Instead, what’s actually being detected are slight perturbations (lasting about 5 days) to the star’s spectrum across multiple wavelengths, which indicates a compositional difference. Since we don’t expect the star to briefly change its composition and then revert back, this is interpreted as seeing gases boiling off the comet as it passes close to the sun.

If so, I’d expect that the particular changes would serve as a kind of “fingerprint” for that particular comet, and be somewhat repeatable the next time it approaches its star. But comet periods can be a lot longer than planet periods (at least in our solar system) so it might take a while to get any repeat signals.

The scientific reason this is interesting is that it can serve to fill in a gap in our knowledge. We’ve seen systems with dusty disks surrounding the star (before planets form), and we’ve seen more mature systems with their planets already formed. We haven’t yet explored the in-between stage in which a lot of material (comets, asteroids) is moving around in the process of forming into large planetesimals. For that reason, astronomers targeted young (type A) stars for the exocomet hunt.

Further, turns out that the technique used wouldn’t work with older/cooler stars. In discoverer Barry Welsh’s talk today at the AAS meeting (he’s giving a press conference tomorrow), he noted that the spectral absorption features they think come from comet outgassing get “narrower and harder to detect in older stars.” I’m not sure of the specifics on this, but it suggests we won’t be able to find exocomets in all of the stars we’ve been studying… at least this way. Astronomers’ innovations will continue to push the envelope!

Will humans ever go to Mars?

I get asked that question a lot. I end up giving two answers: my own wishful dreams, and the less inspiring view of what I think might actually happen.

I recently came across a thoughtful article that agrees with my complicated views on the subject very well. It’s titled “Mission to Mars: Will America Lose the Next Frontier?” After noting the merits of the MSL rover, the article points out the downside of the project: by going almost $1B over its initial cost estimate, MSL has forced the delay or cancellation of other Mars endeavors. (I believe that the article’s note about the cancellation of the Mars 2016 mission is a reference to the 2018 MAX-C mission, a step on the path to sample return, which was canceled. We do have a mission slated for 2016, announced after the article’s publication: the Mars Insight lander.) Similarly, the article notes the terrible impact that the James Webb Space Telescope has had on NASA’s astrophysics program. JWST is NASA’s poster child for mind-blowing cost overruns. Initially estimated at $500M, it’s grown by leaps and bounds and is now estimated at $8B. Both MSL and JWST are sure to deliver rich scientific gains in their respective missions. However, I think this article is correct and fair to note the other efforts that have fallen by the wayside to ensure that these projects are complete.

The main message of the article, however, is the bigger view on what this means in terms of larger, longer-term goals:

“But today, thanks to a combination of budgetary stress, regulatory overkill, and an unfortunate lack of political skill at the highest levels of NASA, the Mars exploration program is in deep trouble. It may be a very long time before the U.S. space agency launches another significant Mars mission.”

Put simply, NASA doesn’t have the budget to send humans to Mars. “Regulatory overkill” refers to a strict intolerance of any NASA failure, no matter how large or small, which necessitates over-engineering (and ballooning costs). Unless something dramatic changes in NASA leadership, political weight, or budgetary windfall, it’s unlikely that our space agency is going to get us there. But all is not lost; Elon Musk is on the job.

What is Io’s lava made of?

Jupiter’s moon Io is very active volcanically:

“A Giant plume from Io’s Tvashtar volcano composed of a sequence of five images taken by NASA’s New Horizons probe on March 1st 2007, over the course of eight minutes from 23:50 UT. The plume is 330 km high, though only its uppermost half is visible in this image, as its source lies over the moon’s limb on its far side.” (Robert Wright and Mary C. Bourke)

But what is that lava made of? What materials lie inside the moon that are being spewed out? We can’t (yet) land on Io and test its lava directly. But we can make some inferences based on remote sensing observations of the lava’s temperature. The temperature carries information about how mafic (magnesium and iron-rich) or felsic (silicon-rich) the lava may be.

The best way to test our ability to deduce composition from orbit is to do it here on Earth, where we do have the opportunity to determine the true composition by sampling the lava on the ground. Scientists Robert Wright, Lori Glaze, and Stephen M. Baloga recently reported a positive correlation between temperature observations from Earth orbit (using the Hyperion spectrometer) and ground composition observations of 13 volcanoes: “Constraints on determining the eruption style and composition of terrestrial lavas from space”. The conclusion for Io is that the lava is so hot that it is likely ultramafic: very high magnesium/iron content.

You can read more about this endeavor (and view more pictures).

See our Earth transit, too!

Perhaps you saw the recent Venus transit of the Sun. But what about an Earth transit?

Obviously we can’t see such a phenomenon while sitting on the Earth itself. But some clever astronomers have done calculations to work out when the Earth would transit the Sun from the perspective of other bodies in the solar system, including the Moon and Jupiter.

“In January 2014, Jupiter will witness a transit of Earth. And we can see it too, the astronomers say, by training NASA’s Hubble Space Telescope on the huge planet and studying the sunlight it reflects.”
(From NBCNews, June 4, 2012)

Using Jupiter as a mirror seems a curious strategy, since the reflected light will also be influenced by the chemical makeup of Jupiter’s atmosphere. However, just as with the hunt for exoplanets, if we can stare at Jupiter for long enough before the transit occurs, we can build a good enough model so its factors can be subtracted out from the Earth+Jupiter signal during the transit. Scientists first plan to test this strategy with a Venus transit that Jupiter will see (Earth won’t) in September of this year. And I’ve seen talk that they used the Moon as a mirror to observe the recent Venus transit from the Earth vicinity — but I haven’t been able to find any images of the result yet. Here’s how it works:

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