Observatories and Instruments

Last week, I took some time away from the never-ending slog that is my book prospectus to watch (via the Internet) the liftoff of a Delta II rocket with the Aquarius/SAC-D spacecraft on board. Tense times, but everything went as planned and early telemetry suggests the Aquarius observatory is in good shape.

When I first heard about Aquarius, I thought I was looking at the wrong project description. Measuring the salinity of the Earth’s ocean? NASA? From space? Yes, on all counts. Aquarius was designed to measure Sea Surface Salinity (SSS) as it varies over time and according to region. The project team hopes that by tracking SSS from orbit, they will be able to detect variations in the Earth’s water cycle.  In the end, they hope to build a more complete model of the interrelationship between runoff, the freeze-thaw of sea ice, and evaporation/precipitation over the ocean.

Schematic diagram of Aquarius/SAC-D satellite. Image credit: NASA

How will the observatory accomplish these measurements?  The spacecraft’s primary instrument, which was contributed by Argentina’s Comisión Nacional deActividades Espaciales (CONAE), consists of three passive microwave radiometers that are super sensitive to salinity (1.413 GHz; L-band; this roughly correlates to 1/8 teaspoon of salt in a gallon of water). The instrument also contains an active scatterometer that measures ocean waves that affect the precision of the salinity measurement. Aquarius is projected to spend three years measuring SSS in 7-day cycles, after which time the data will be used to theorize on numerous pressing issues:  how ocean currents effect salinity transport; SSS impact on tropical climate models and El Niño; SSS impact on oceans ubsurface dynamics; ice-ocean interaction; processes that maintain the ocean’s salinity; and so on.

As usual, NASA has provided an abundance of interpretive aids and flashy images.  You can download a project overview in print/text, or just watch the overview video. If you’re in a hurry, you can take a quick look at the diagram of the project’s research priorities and anticipated scientific outcomes. If you’re talking about Aquarius in the classroom, you can print out educational wall posters in English and in Spanish.  The gallery for the project has schematics for launch, stowage and full deployment of the satellite’s instruments. I anticipate some new additions to this section now that the observatory is in orbit above Earth’s oceans.

Astrolabe

My doctoral research focused on instruments derived from Ptolemaic principles. However, one of the observatories on which I based my project was also associated with an astrolabe foundry during the first half of the 18th century. Technically, astrolabes aren’t Ptolemaic. The stereographic projection on which the astrolabe depends was first described by Hipparchus c. 150 BCE, some 250 years before Ptolemy described it in his Planisphaerium (2nd c. CE). All the same, I could see needing to explain functionality of the instrument to my dissertation committee, so I started keeping track of the better resources on the instrument in my research notes.

A few fragmented astrolabes exist at one of my research sites, mostly in the form of tympani missing the necessary moving parts (the rete, rule and alidade) to make them useful. Staring at the tympani didn’t do me much good and my reading made sense in theory, but lacked practical examples. I really didn’t want to find myself standing in front of my examination committee, stammering for an answer when they asked me to explain how to use an astrolabe to determine local solar time.

Enter Adler Planetarium, the sole purveyor of Roderick S. Webster’s Astrolabe Kit, the solution to my problem.* I’m a great believer in hands-on learning experiences.  If you want to know how something works, try building it for yourself.  Or try putting it back together after you’ve taken it apart. It works with construction methods used in residential design and, as it turns out, it also works with astrolabes.

Front of Astrolabe, showing tympanum, rete and rule

Webster’s kit is a little pricey (around $20) but I think I’ve gotten twenty bucks worth of entertainment and education out of it.  It took me about three days to build the thing because of its complicated glue demands. Without glue, it would’ve required 30 minutes, tops, including sanding the rough edges and assembly. Learning how to use the thing is been a different matter. I’ve spent (literally) hours sitting at my desk working through the various problems it’s supposed to help me solve. Local skies have been persistently overcast, so I’ve been using Stellarium to help me determine the altitudes of various stars (if the skies ever clear up, I’ll be able to sight them through the rule on the back of the astrolabe. IF THE SKIES EVER CLEAR UP.). I’m currently using the tympanum set up for a latitude of 42 degrees.  That gives me the skies about Barcelona (41 23 N), Marseilles (43 20 N), Rome (41 54 N), Sofia (42 40 N), and Vladivostok (43 10 N) with which to play this evening.

Back of Astrolabe, showing rule

And playing is what I’ve been doing. Sidereal time, solar time, time from the sun, sunrise and sunset, the positions of the moon and planets, rising and setting of a few major stars… Some calculations are working better than others, but I’m not quite sure why. Once I’ve determined the latitude of a star and set the rete, I never move it, no matter what problem I’m trying to solve. It’s a bit of a mystery as to why one answer is immediately obvious and the next isn’t, but it’s also a bit of fun.

*N.B. I spent an outrageous amount of time trying to find the kit in the newest version of Adler’s online store, yet still failed to locate it. You might have to call them if you really want one.

Last week, while my attention was elsewhere (no one can prove I was watching KLF videos on YouTube), NASA announced the results of an EPIC space-time experiment. The World of Warcraft language is all theirs, but I have to admit that EPIC is an appropriate descriptor in this case. According to analyses of data returned by Gravity Probe B (GP-B), Einstein was correct.  The earth exists within a 4-dimensional space-time fabric that behaves precisely as predicted:  the mass of the earth dimples space-time and the movement of the earth distorts the dimple, creating a vortex or swirl in space-time.

GP-B is a nifty little instrument. As the graphic below shows, the orbiting spacecraft encapsulates a 9′-tall, 650-gallon dewar flask (thermos)  filled with cooled liquid helium. Before launch, the cigar-shaped probe was inserted into the helium along the central axis of the flask.  The instrumental component of the probe is the Science Instrument Assembly (SIA), which is composed of a telescope and a quartz block housing four gyroscopes.

Gravity Probe B Payload Components. Image courtesy of NASA.

In 2004, the spacecraft, complete with SIA, was launched into a polar orbit 642 km (400 mi) above the Earth.  After the satellite reached orbit, the telescope and the spin axes of the four gyroscopes were aligned with a pre-designated star. The goal was to keep the telescope aligned with the star for a year without making similar axial corrections to the gyroscopes.  After a year, the (postulated) precession change of the spin axis alignment of the gyros would be measured in respect to the plane of orbit (the geodetic precession) and the plane of the Earth’s rotation (frame-dragging precession).

Scientists are interested in the geodetic effect because it is an effective measure of how far the Earth is warping its local space-time.  The axial drift is incredibly small (0.041 arcseconds over a year, with one arcsecond equalling 1/3600th of a degree), but the fact that it was measured at all indicates that the axis was tracing the curves introduced into the fabric of space-time by the mass of the earth.  Most simply, the measure of geodetic precession reflects the “dimple” in space-time.

Scientists are even more interested in  the frame-dragging effect, however.  The idea that massive celestial bodies drag their local space-time around with them as they rotate was proposed about ninety years ago, almost immediately after Einstein gave us his theory of general relativity. If it’s true that space-time is dragged a bit during rotation, we would expect to see evidence not of a perfect dimple, but more of a twist, like a small tornado.  It works on paper, anyway, but we’ve lacked the ability to measure the drag.

Until Gravity Probe B, that is.  This is actually the spectacular part:  the design team managed to encapsulate the probe in a drag-free satellite that protected the gyroscopes from disturbance as it moved through the planet’s outer atmosphere. Moreover, they designed a device that measured the spin of the gyroscope without actually touching the them (which would have disturbed their motion and made the results useless).  While the results might indeed be EPIC, I have to admire the design process even more. Sometimes it can be difficult to get three people in the same room working collaboratively. Gravity Probe B comes to you courtesy of teamwork between NASA, Lockheed Martin, Stanford University, and King Abdulaziz City for Science and Technology in Saudi Arabia. That is EPIC, indeed.

Read more on the Gravity Probe B technology here and here.  Check out the GP-B in a Nutshell posters for explanatory graphics