17 March 2011

Where is north?

Where is north is actually intensely tied to the question of What is a day?.  At least we wind up defining it in much the same way(s) as we define the day.  In the previous post, I gave a definition for north/south.  Namely, the line of a shadow cast by the sun at solar noon (itself define by the fact that it's the shortest shadow of the day) is north/south.

As happened for the day, we find our most accurate definition from examining the stars other than the sun.  The 'pole star' isn't actually one that we use for this.  It's almost a full degree away from the actual pole of the earth's rotation.

What we do instead is look for day to day differences in the location of stars passing overhead or nearly so.  This approach dates farther back than Seth Carlo Chandler, in the 1890s.  But we'll be coming back to Chandler.  If the earth has wobbled a little to the north, then the star will pass the zenith a little to the north of where it did yesterday.  If you've got a good telescope and other instruments, you can observe this to pretty good precision.  Chandler was working with accuracy of 1 second of arc or somewhat better for single measurements.  Because of the power of using multiple measurements he was able to examine earth wobbles that were less than 0.1 seconds of arc.

This turned out to be quite useful, as the earth wobbles by about 0.3 seconds of arc.  It's how he discovered what was promptly called the Chandler Wobble.  This translates to about 3 meters motion in where the pole is.

The orientation of the earth is, as with the rotation rate, tied to where the mass is and where it moves to.  The earth's orientation is believed to have changed by some millionths of an arc second due to the earthquake.  The variations of a few tenths of a second of arc are caused by ... other things.  Atmosphere and ocean circulations are what I'm most concerned about, but also the earth's inner core, and the moon, and ....  It's a messy business.

As for the length of day, your scientific source for observations is the International Earth Rotation Service.  Which, itself, owes something to Chandler.


David said...

Nice. Might a brief discussion of the discussion between angular momentum and angular velocity be in order? I presume that the Earth's angular momentum varies less than it's angular velocity? And that it's the moment of inertia that's changing (via ocean and air and crustal movements)?

Penguindreams said...

The What is a day? post mentioned some of this on angular momentum and angular velocity. There'll be more as I get in to this set of notes.

The earth's angular momentum is a constant -- unless acted on by something from outside. So earthquakes, atmospheric and oceanic circulation, core rotation, ... don't change that. But they can and do change the earth's moment of inertia, which then mandates a change in its angular velocity. imback mentioned some of this and gave a couple of links in a comment on the 'What is a day' article.

On the other hand, the earth _is_ acted on by things from outside. Mostly the moon and sun. They do change the earth's angular momentum without having to affect its moment of inertia. The size of this is not small compared to the changes from earthly processes.

Kooiti Masuda said...

International Earth Rotation Service was formerly International Latitude Service, which started observation in 1899 at Gaithersberg, Maryland and Mizusawa, Japan among others.

Precise description of the polar motion by Hisashi Kimura (who led observations at Mizusawa) was a moment of demonstration that the Japanese can substantially contribute to the international scientific enterprise.

Those stations share the same latitude (39 deg. 08 min.), which fact is helpful for me to find locations of comparable latitudes in the USA and Japan.

Mizusawa is located in the interior of the Tohoku (northeast) region of Honshu Island. The specific position along the parallel was chosen surely because the east and the west coastal locations are cloudy in summer and in winter, respectively.

Presently the Mizusawa observatory is a branch of the National Astronomical Observatory of Japan, specialized in VLBI (Very Long Baseline Interferometry).

The web site of the Mizusawa observatory says (in Japanese) that the direct damage due to the recent earthquake was not large there. (Large damages occurred along the east coast.) But, infrastructure in the whole Tohoku region is disrupted, and it will take some time for normal opreations to resume.

Kooiti Masuda said...

Excuse me, I misspelled the name of the location in Maryland: it is Gaithersburg.

Another tribia: Mizusawa is also known as the hometown of a statesman Shinpei Goto, who led the reconstruction of the City of Tokyo after the earthquake of 1923.