On a sandy, arid plain, near the Rincon Mountain Visitor Center of Saguaro National Park, tucked 

in among brittlebush, creosote, and other hardy 

desert plants, is an unusual type of observatory—a 

small unmanned station that is used for monitoring the Earth’s variable magnetic field. Named for 

the nearby city of Tucson, Arizona, the observatory is 

1 of 14 that the Geomagnetism Program of the U.S. Geological Survey 

(USGS) operates at various locations across the United States and Territories. Views of the Tucson observatory are shown in figures 1 and 2.

Data from USGS magnetic observatories, including the Tucson 

observatory, as well as observatories operated by institutions in other 

countries, record a variety of signals related to a wide diversity of 

physical phenomena in the Earth’s interior and its surrounding outerspace environment. The data are used for geomagnetic mapping and 

surveying, for fundamental scientific research, and for assessment of 

magnetic storms, which can be hazardous for the activities and infrastructure of our modern, technologically based society (for example, 

Love and others, 2008). The USGS observatory service is an integral 

part of a U.S. national project (National Science and Technology 

Council, 2015) for monitoring and assessing space weather hazards.

History

The history of the Tucson observatory can be traced back to June 

1909, when President William H. Taft signed an executive order setting 

aside a parcel of Federal property outside the town of Tucson to be 

used by the Coast and Geodetic Survey of the Department of Commerce and Labor for a magnetic observatory (Hazard, 1913). Soon 

thereafter, construction began on an office building and a building for 

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2017–3035

June 2017

Figure 1. Panoramic view of the Tucson magnetic observatory in 

Saguaro National Park.

Figure 2. Tucson observatory’s electronic building with 

surrounding flora.

Tuscson Magnetic Observatory

Magnetic Monitoring in Saguaro National Park

analog magnetometer systems. The first hourly measurements 

of geomagnetic variation at Tucson were reported in November 

1909. In the 1970s, agency reorganizations within the Federal 

government led to a reassignment of responsibility for operating 

the observatories to the USGS of the Department of the Interior. 

Over the years, the city of Tucson grew, and commercial and residential developments eventually surrounded the 

Tucson observatory site, interfering with sensitive magnetic 

instruments. In 1989, the city of Tucson agreed to support 

the USGS in the search for a new observatory site. A site was 

identified within Saguaro National Park, and modest facilities 

were constructed. In January 1996, data from the new observatory site were first reported. The USGS Tucson observatory 

operates to this day in Saguaro National Park, under terms of a 

special use permit with the National Park Service. The former 

observatory site is now part of the city of Tucson’s Udall Park.
References

Hazard, D.I., 1913, Results of observations made at the Coast 

and Geodetic Survey magnetic observatory near Tucson, 

Arizona, 1909 and 1910: Department of Commerce and 

Labor, p. 1–59, 9 figs.

Love, J.J., Applegate, David, and Townshend, J.B., 2008, 

Monitoring the Earth’s dynamic magnetic field, U.S. 

Geological Survey Fact Sheet, 2007–3092, 2 p.

National Science and Technology Council, 2015, National 

space weather strategy: Washington, D.C., Executive Office, 

National Science and Technology Council, p. 1–13.

More Information

USGS Geomagnetism Program, https://geomag.usgs.gov

Saguaro National Park, https://www.nps.gov/sagu/index.htm

Data

The compass is a familiar device used for measuring the horizontal direction of the Earth’s magnetic field. A subset of the 

measurements made by a magnetic observatory can be expressed in terms of declination, the difference between the local direction 

of the compass needle and geographic north. It is interesting that the direction of the needle changes in time. Figure 3 shows a record 

of the declination at the Tucson observatory since 1909. Slow variation in declination, shown in blue, is caused by fluid motion in 

the Earth’s deep interior—the site of a dynamic process that generates most of the geomagnetic field. More rapid variation, shown in 

gray, is caused by transient change in the Earth’s surrounding space-weather environment, especially during magnetic storms. The 

offset in declination corresponding to moving the observatory in 1996 is because of a difference in the local magnetization of the 

rocky crust beneath the observatory site. 

14

13

12

11

10

D (degrees east)

1920 1940 1960 1980 2000

Observatory

moved

Year

Figure 3. Hourly (gray) and annual (blue) values of magnetic declination recorded at the Tucson (Saguaro) observatory, 

1909–2013. The offset at 1996 was caused by relocating the observatory to Saguaro National Park.

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20173035

By Jeffrey J. Love and Carol A. Finn, U.S. Geological Survey; and 

Yesenia C. Gamez Valdez and Don Swann, National Park Service