Ukiah, California Latitude Observatory

© Brooke Clarke 2007
                  Telescope of the type used in the Ukiah Latitude
                  Observatory Ukiah Latitude
                  Observatory clock face
Ukiah Latitude Observatory Chronograph
Straddle Level for Vertical axis
                Latitude Observatory Striding Level

Ukiah Latitude
                  Observatory clock

Ukiah Observatory
                  Roll Off Roof in Shade
Roll Off Roof Observatory Building
protected from sun.

Two out buildings to the right.

Viewed from Observatory Ave.
Ukiah Observatory
Roll Off Roof Observatory Building
protected from sun.

Astronomer's house to the left.

Viewed from Observatory Ave.
Ukiah Observatory
Astronomer's house.

Viewed from Observatory Ave.
Ukiah Observatory
Out buildings.

Viewed from Observatory Ave.
Ukiah Observatory
Out buildings, Roll Off Roof Observatory, house.

Viewed from Luce Ave.
Ukiah Observatory
Roll Off Roof Observatory, house.

Viewed from  Luce Ave.
NOAA 1930? photo of first astronomer's house & observatory.
It looks to be much closer to the hills than the current location?
The first house is now on an adjacent property.
Front of
                First Ukiah Latitude Obs Astronomer's house

Ukiah Latitude Observatory Looking South
Popular Science Monthly Nov 1909
                Latitude Observatory 1909 Looking South
Note that the telescope is sitting on a pedestal that's waist high.  The reason for
the North facing door is so that when the telescope is horizontal it can see a North
alignment mark at the top of a waist high concrete pier at the far end of the property.

Zenith Telescope
Popular Science Monthly Nov 1909
Figure 4
                  Latitude Observatory Zenith Telescope
When scope is rotated so that horizontal axis is East-West
the scope moves in the meridian plane.
There are stops on the lower azimuth circle that can be set to 180 deg.
If the scope is pointing 10 deg East of meridian and the scope
rotated about the vertical axis to the other stop 180 degrees away
then it will be pointing 10 degrees West of the meridian
allowing the difference between the zenith angles to be
measured using the eyepiece micrometer.

Causes and Types of Polar Motion
Photographic Zenith Tube (PZT)
Plate Tectonic Motion
Ukiah Astronomers
Latitude Observatories 
    Zenith Telescope
    Magnetic Repeat Station
    Plot Plan & Markers
Talcott Method
Coordinates of North Pole
Lunar Ranging


facebook web page.

Starting in 1899 observations were made to determine the location of the Earth's rotational axis, i.e. finding the latitude.  To do this a handful of observatories were spread pretty much equally around the globe at 39 degrees 08 minutes North Latitude.  They sequentially observed the same set of about 60 star pairs each night measuring the zenith distance and time of meridian crossing using a Vertical Zenith Telescope (VZT).

In the 1977 NOAA report (See References below) they say "Probably no other project in modern science has continued for so long unchanged in purpose, equipment, or technique and has managed to produce such a large volume of high-quality data."

The book Latitude by Carter & Carter ( ISBN 1557500169) has a good description of the problem of finding the Latitude and the brilliant work of Chandler and Newcomb.  Ukiah was the location of one of the small number of Latitude Observatories  (Wiki) which measured their latitude nightly in order to help define the wobble of the Earth's poles.  The observatories were started in 1899 and decommissioned in 1982.

The book Longitude by Dava Sobel describes the 40 year effort (1730 - 1770) that John Harrison (wiki) went through to develop the marine chronometer.  There's also a movie based on  the book called Longitude.  What Harrison accomplished is fantastic both in terms of a marine chronometer but also in terms of horology.  For example he was the inventor of the temperature compensated pendulum.  But once accurate balance wheel clocks or better quartz clocks or GPS navigation receivers are available the problem of finding the longitude goes away.

Alfred L. Loomis (wiki) got half a dozen of the Shortt precision pendulum clocks (Wiki) and discovered that they were effected by the Moon's gravity and was the first to publish this limitation of pendulum clocks (Wiki) in 1931.  The book Tuxedo Park by Jennet Conant has details of that.  Some details of the Moon and Sun gravity tides is on TVB's web page Lunar/Solar Tides and Pendulum Clocks.

The movement of the North pole caused by the wobble of the Earth was a much larger problem that took longer to solve than the longitude problem.  Many astronomical observations can not be made without a knowledge where the North pole is located (i.e. where the spin axis of the Earth is pointing in space).  The Latitude Observatories operated for almost a century.  It's only been very recently that a connection with El Nino and the wobble has been found.  The problem with the wobble is that it's not predictable as of 2007. 

There are a number of modern ways that the orientation of the Earth is currently being measured.  This is coordinated by the International Earth Rotation  & Reference Systems Service in France.  They make use of such methods as laser range finding to Earth satellites and the Moon and Very Long Base Line Interferometry.

To see the Ukiah sky check out my Sky-Weather-Astronomy web cam.  If it's not cloudy at night you can see dozens of stars on line, millions if you are here.  The zenith is close to the top center of the image and Polaris is above the tree on the right.

Causes and Types of Polar Motion

There are forces acting on the Earth that cause it to orbit the Sun and translate in space that are not related to to the latitude problem.  They result in the precession (Wiki) and nutation (Wiki) of the poles.

The unbalanced forces that can effect the latitude come about because the distance between the North and South poles is shorter than the diameter at the Equator.  This gives rise to a Euler period of about 300 days if the Earth is assumed to be rigid or the period Chandler predicted of about 305 days.  Chandler Wobble (Wiki) is added on top of the Euler spiral and is caused by the circulation of air and water, earthquakes, continental drift and changes in the Earth's interior (2014) and El Niño (Wiki) ocean currents.

If El Niño has an effect them maybe global warming will show up as a perturbation of the Earth in a way that can be seen using precision astronomy or time?  For example when a large volcano erupts it changes the distribution of mass on the Earth, in a way that's similar to an ice skater extending her arms, that causes a change in the period of the earth's rotation.

Photographic Zenith Tube (PZT)

On some of my other web pages there's information about the PZT but in reference to it's use to determine the time based on the Earth's rotation.  But in the D.O.C. U.S. Coast and Geodetic Survey Special Publication No. 27, 1915 "Latitude Observations with Photographic Zenith Tube at Gaithersburg, MD."  is a report on the use of the PZT. (atitudeo00uscouoft.pdf)
In this paper problems with earlier observation methods are described.

Plate Tectonic Motion (Wiki)

"The secular variation of longitudes and plate tectonic motion" describes how the data from the Latitude Observatories were used to test the plate theory.  This implies that the time of star meridian crossing was measured along with the zenith angle.

Ukiah Astronomers

1899 to 1903 Frank Schlesinger (Wiki)
Cal Micrometer
1903 to sep 1907 Sidney D. Townley (Wiki)
April 10, 1867 - March 18, 1946
San Francisco Earthquake damage in Ukiah:  5:12 AM - April 18, 1906

Assistant Professor of Applied Mathematics at Stanford in 1907 - 1946

charter member & Editor of
 Seismological Society of America  1911 to 1929
1907 to 1911
James D. Maddrill


1918 to 1922
Ferdinand Neubauer

Frank Schlesinger testing of automatic Zenith Camera at Allegheny Observatory
1 Jly 1948 - 30 Jun 1949
Leonard F. Caouette
1988 star pairs observed (1611 in Gaithersburg)
William F. Meyer

Latitude Observatories

The observatories built specifically for the International Latitude Service were all located very near 39 degrees 8 minutes North Latitude.  The observation method was as developed by Talcott in 1834 which requires observing pairs of stars where the distance is balanced on the North and South sides of the zenith and the time of culmination is reasonably close.  This allows making better measurements than prior methods.  It requires that the observatories be located all around the Earth and very close to the same latitude so that they are looking at the same star pairs thus allowing for error corrections.

N 39:08:12.51 W 77:11:55.85 Gaithersburg, Maryland USA
N 39:08.3 W 84:25.4 Cincinnati, Ohio USA built for political reasons, too close to Gaithersburg to be of much value, closed early in the program
N 39:08:14.26 W 123:12:42.54 Ukiah, California USA
N 39:08.1 E 141:07.9 Mizusawa Japan National Institutes of Natural Sciences National Astronomical Observatory of Japan Mizusawa VERA Observatory
N 39:08.0 E 66:52.9 Kitab Uzbekistan
N 39:08:13.76 E 8:18:41.90 Carloforte Italy


Zenith Telescope (Wiki)

The telescope is made specifically for measuring the angle between straight up and a star near the zenith. 

American History - Zenith Telescopes - Julius Wanschaff made the Ukiah scope. 108 mm ( 4 1/4") aperture and 1300 mm (51.8") focal length (f12).    This is a much higher quality instrument than the earlier Troughton & Simms zenith scopes.  I've heard that the angular accuracy was good to 0.1 arc seconds.  This is orders of magnitude higher than today's high end telescopes.  I suspect they were able to do that because it's a very specialized instrument.

Note that Wild patented a theodolite in 1907 and by 1926 was making 1 arc second theodolites (T2) and later made the T4 good to 0.1 arc seconds.  But the Ukiah observatory was making measurements good to 0,01 arc seconds in 1899!   A feat that even today is fantastic.

The astronomer would press a button when the star crossed the horizontal line to mark the chronograph.  But the key operation would be to adjust the pointing of the scope to measure the zenith angle of the star as it rises to maximum height and then recedes.

Ukiah Latitude
        Observatory Base for Vertical Zenith TelescopeThis is the concrete base for the vertical zenith Telescope.  It's a waist high concrete pier surrounded with wood that matches the interior walls.  You can see three light colored circles where the feet of the telescope sat.  The observer sat in a chair and looked into an eyepiece that was maybe half a foot above the concrete.  In front of the observer was an electrical switch that when activated would make a mark on the chronometer paper.


Strasser & Rohde 599 (German collector web page) most likely an Invar rod pendulum with temperature compensation with lens shaped bob.
The small top hand is seconds, the sweep hand is minutes and the bottom hand is a 24 hand.  Set to run at sidereal time not standard time.  The purpose of the clock is to know which star will be visible next.   The clock would have an electrical switch that would close for a short time once each second used to make seconds ticks on the chronograph.

Later the Heathkit Most Accurate digital clock was used (photo in old local newspaper at Historical Society).  Note that the clock is to tell the astronomer approximately when a star needs to be observed and then to record the actual meridian crossing time as well as zenith angle.  Note that the position of the stars was not well known then because no observatory knew it's latitude because of the precession (Wiki, axial precession) and wobble of the Earth (Wiki: Chandler Wobble, Polar Motion).  By having all the latitude observatories observe the same star pairs the location of those stars could be refined.  Note of each star pair one star was to the East and the other to the West of the meridian.

Sidereal Time (Wiki)

Sidereal time is based on star meridian transits instead of solar transits as is used for standard time.  Since both are based on an observer on Earth the difference between the two time scales is exactly one day per year since the Earth is revolving around the Sun and it's own axis.

Local Sidereal Time (Wiki) is adjusted for the longitude of the telescope.

The location of a star is commonly specified by it's Right Ascension (Wiki) and declination (Wiki).  The R.A. is the local sidereal time when the star transits the meridian. 

So for a night of observations there will be a star list ordered in R.A.

For example the star Vega is a magnitude 0 star R.A. 18:27:12,  dec 37 deg 47 min 18 sec North  so when the sidereal clock says the time is 18 hours 27 minutes and 12 seconds it will be very close to directly overhead in Ukiah, CA.  On 21 June 2007 at 1:54:32 local time Vega will be very near the zenith.  It's expected zenith angle will be 0 deg 29 min 0 seconds  and to the South of the meridian.


It's not clear to me why the chronograph would be needed, other than as an aid to setting the clock.
4 Jun 2009 - The NOAA May 1977 paper on polar motion mentions that the latitude observatory data was used to see if they changed longitude.  That implies that the time of meridian crossing was recorded.  If so then the observatory clocks were not set by looking at the stars, but rather by reference to the USNO time.  Up until the mid 1960s that could be done using the Western Union time service (see my Self Winding Clock Co web pages) and after the mid 1960s by using the Heathkit GC1000 radio clock.  Since neither the WU time service nor the Heathkit GC1000 are sidereal time pieces they would be used to set the local pendulum clock which might have kept sidereal time.  Or, if it kept regular time a mathematical conversion would be made.


The buildings for all the latitude observatories may be the same (very similar) in design.  In any case the Ukiah building and the Gaithersburg were made from the same set of plans but the as built configuration was different and changed over time.

A large part of the building design relates to how it cools when the roof is rolled back and how it heats during the daytime when the roof is closed.  This is extremely important since thermal gradients will effect the accuracy of the telescope.  There is a thermometer on the wall of the observatory and a chart for converting between F and C.


Sidney D. Townley was the resident astronomer at the Ukiah Latitude Observatory on April 18, 1906 and felt the San Francisco earthquake.  I'm guessing it made a big impression on him because when his assignment at Ukiah was up he moved to Stanford University and in 1911 was one of the founding members of the Seismological Society of America acting as the editor of their key publication.
Ukiah Latitude
          Observatory Seismometer ConcreteIn the basement of the astronomers house there's a room maybe 16 by 14 feet and in the rear corner of the room the top of a concrete block about  8 x 8 feet on a side is sticking up from the floor about two feet.  There is a gap between the concrete mass and the building floor to isolate it.  It was used for developing seismometers.  (Photo by PaulK 30 May 2009)

H. G. WROCKLAGE, Installation of McComb-Romberg tilt-compensation horizontal-component seismometers at the International Latitude Observatory, Ukiah, California
Bulletin of the Seismological Society of America, January 1, 1934; 24(1): 69 - 71.
Arnold Romberg (Univ. of Texas)
patent 2293437 Force Measuring Device, Aug 18, 1942

Provisional result of the work of the international latitude service in the North Parallel + 39° 8’ during the year 1933
History of the Seismological Society of America -

UKI 39.1372 -123.2106 0.199 Ukiah, NEIC
Is this the Latitude Observatory? The Lat is the same (39.1372 vs. 39.137294) so yes.
The USGS shows UKI as an open seismic station, probably a typo.

Magnetic Repeat Station

The Plot Plan shows a number of survey markers that suggest that a Magnetic Repeat Station was located at the observatory site.  There are magnetic observatories where the instrumentation is permanently installed and there are repeat stations where the instruments are brought to the site once every few years.

IAGA Guide for Magnetic Repeat Station Surveys, by L.R. Newitt, C.E. Barton, and J. Bitterly, 1997, ISBN: 0-9650686-1-7 -

Note because the latitude observatories were probably the places on Earth where their location was known to an extremely high precision I expect they would also be home to other scientific investigations where they need to know the location.

Plot Plan

The plot plan made in 1991 shows the boundaries, North-South direction and a number of survey markers.

Ukiah Latitude Observatory Plot Plan 1991 Magnetic Station Azimuth Mark
Ukiah Latitude Obs Mag Sta Ref

RM 3
                Latitude Obs Magnetic Station Ref Mrk 3
Meridian House (being restored)
                Latitude Obs Meridian House Marker
USGS Magnetic Station
                Latitude Obs Magnetic Station
                Latitude Obs Magnetic Station Ref Mrk 4c
RM5 NGS Data Sheet
PID - KT2010
                Latitude Obs Magnetic Station Ref Mrk 5
                Latitude Obs Mag Ref Mark 1
Magnetic Station Azimuth Mark
                Latitude Obs Mag Sta Ref Mark

The Northern most marker is a concrete post with an angle iron bracket at the top and labeled "Magnetic Station Azimuth Mark".
It's not clear to me what this is about.  Maybe there was a magnetic filed station here?
Note that along a North-South line through this marker is the "Ukiah Magnetic 1925" marker and a notation showing a fifteen foot nominal radius.  There are 5 other survey markers all radiating out from the Ukiah Magnetic 1925 marker labeled: RM1, RM2, RM3, RM4 and RM5.  Maybe for Relative Magnetic?
Marker photos by PaulK.

NGS Data Sheets at the Ukiah Latitude Observatory

KT1197 - NAD 83(1986)- 39 08 15.06464(N) 123 12 43.35952(W) AD(1984.00) 2
                           STAMPING: 651.928 B 105 1932
                           The magnetic marks are listed on this data sheet.

KT1198 - NAD 83(1986)- 39 08 14.10437(N) 123 12 42.15978(W) AD(1984.00) 3
                        The top and extreme northwest corner of the concrete base supporting the zenith telescope and about 4 feet above the floor.
PID    Reference Object                     Distance      Geod. Az  
KT2010 UKIAH MAGNETIC AZ MK                 99.828 METERS 00000 
NAD 83(1986)- 39 08 18.30181(N) 123 12 43.35939(W) AD(1984.00) 2 KT2024 UKIAH MAGNETIC RM 3 42.632 METERS 01959

KT2023 UKIAH MAGNETIC RM 4 31.887 METERS 02829


KT2011 UKIAH LATITUDE 1925 41.413 METERS 13547


DB6008 UKIAH MAGNETIC RM 1 15.193 METERS 22349

DB6009 UKIAH MAGNETIC RM 2 15.925 METERS 30730


Ukiah Latitude Obs
        Indoor Temperature Chart & Thermometer bracketThe bracket probably held a thermometer and to it's left is a conversion chart between F and C.  The chart was not typed with the minus signs, but they have been added by hand.  It also is missing the decimal point for the tenths of a degree C making it awkward to read.  There were also thermometers outside on either side of the building that were recorded at about the center of the observation run each night.

Photo by PaulK 30 May 2009

Talcott Method

Explained in Topographic, Trigonometric and Geodetic Surveying: Including Geographic, Exploratory, and By Herbert Michael Wilson, 1912 J. Wiley & Sons 932 pgs.

Talcott's method took into account that the star catalogs then available (maybe a hand full) had errors for some stars.  By observing a group of stars (maybe a couple dozen) in a night and by pairing the stars so that one was North of the zenith and the other was South of the zenith not only instrument errors but also star catalog errors could be detected and corrected.

The same star list was used by all the observatories.  This allows for further error detection and reduction and is why all the latitude observatories are very close to 39:08 N.

The stars are in pairs where each star in a pair is very close to the same zenith angle.  The scope is set for the average zenith angle of the pair using the vertical circle and vernier and the scope is set East or West depending on which star culminates first.  When that star crosses the meridian the micrometer eyepiece is set to measure how far from the central angle the star is.  Then the scope is revolved 180 degrees about the vertical axis and when the second star crosses the meridian the micrometer eyepiece is again used to read how far it is from the central angle of that pair.  The micrometer readings are much more accurate than an angle read from the vertical circle and is the key benefit of the Talcott method.

Coordinates of North Pole

The current data is at Earth Orientation Center - Polar motion for the last 365 days -

Lunar Ranging

This is a modern part of determining the period of the Earth's rotation as well as other astronomy.  The idea is to measure the time it takes a pulse of laser light to go from the Earth based observatory to one of the retro reflectors on the Moon and come back.

It turns out that, just like the latitude observatory, great care is needed to make precision measurements.  In the case of the lunar ranging the Earth's gravity tides that cause the ocean water level to move up and down also cause the bedrock at the top of mountains to move up and down (Wiki).

There are a couple of ways to measure the movement of the telescope used to make the lunar distance measurements:
1) precision GPS - but it also measures all the changes, not just the change due to earth time
2) a gravity meter, like that GWR that's used for the APOLLO system, allows correcting the telescope position to account for the earth's tide.
3449956 Force measuring instrument, Goodkind John M, Prothero William A, Jun 17, 1969, 73/382.00R, 73/514.18
5204568 Superconducting bearing for borehole and survey gravimeters, Robert L. Kleinberg, Douglas D. Griffin, Richard J. Warburton, Gwr Instruments, Apr 20, 1993,
                310/90.5, 33/366.11, 324/346, 73/382.00R - 
Apache Point Observatory Lunar Laser-ranging Operation (APOLLO)
The Basics of Lunar Ranging
Wiki- Lunar Laser Ranging experiment
ScienceShot: Decades-Old Soviet Reflector Spotted on the Moon


Flickr - survey marker photos -
National Parks Service - Astronomy and Astrophysics - Gaithersburg Latitude Observatory -
NOAA (National Oceanic and Atmospheric Administration) -
Gaithersburg International Lattitude Observatory - Wendy Woodland

Martin Bradley - The person most responsible for saving the observatory - photo sequence of THE oak tree with star background - Telescope used in Ukiah -
Paul K - Photos taken during 30 May 2009 tour of buildings.
California's First Observatories. By Peter Abrahams.  Ukiah observatory had determined it's latitude and published it in 1897.
Flamsteed Astronomy Society -
NWAC - Precision Regulator Clocks Gallery - Strasser & Rohde

Popular Science Monthly - Nov 1909 "The Shifting of the Earth's Axis" by Dr. Sidney Dean Townley (Stanford univ) - mentions Euler's formula of 1765 giving a 306 day period if the Earth was rigid.  The actual period is more like 427 days.  But the axis does not repeat period to period.  As a possible explanation:
"Ah see by de papers dat de urf's axis am a wobbling an' dey dunno wat fo'.  But ah know wat makes de urf's axis wobble.  Do you see all dis oil dese men am a takin' out of de urf?  Well wat do you spose de good Lord put dat oil in dere fo'?  Wy to grease de axis wif, of course, an' when dey take it all out, wat else can de axis do but to wobble." Attributed to a Preacher in the oil region of Texas, but probably the Townley's humor.

39315 Altitude Instrument, March 29, 1881, S.C. Chandler, Jr. - aka Chronodeik or Almucantar - a telescope coupled to a pendulum so that it points straight up.

"The Observatory" of the Royal Astronomical Society, 1899 (Vol XXII) - pg 445 - Book Review "A Treatise on Instruments" published in German 1899 mentions the Chronodeik is in the book along with typical data.

The SAO/NASA Astrophysics Data System - The Astronomical Journal - No. 1180 page 219 " - report for year: 1 Jly 1948 - 30 Jun 1949

Systems for the Determination of Polar Motion
, May 1977, NOAA - Continuing the Lat Obs operations mainly as astronomical & navigational reference frame also may link to/from  earthquakes.  Plan to continue for 50 or 100 more years.  In 1898 the International Geodetic Association (IGS) created the International Latitude Service (ILS) (the name changed in 1962 to International Polar Motion service (IPMS)) of 6 stations all located at 39:08 North to observe 12 groups of 12 stars.  Later the stations at Cincinnati dropped from the service and Tchardjui was replaced by Kitab.  Gaithersburg was non operational from 1916 to 1932, but Misuzawa, Ukiah, and Carloforte gave almost uninterrupted service.  Also starting in 1962 more observatories were added at various latitudes bringing the total to about 50 observatories observing not only stars but also the Moon, artificial satellites and quasars using optical, laser and radio methods.

The observatory locations given in this paper differ from those shown above probably due to the reference frame used.  The paper was pre global navigation and GPS so used an older less accurate country specific reference frames.

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