Joel Stebbins Observed Linné Crater During Feb. 8, 1906, Lunar Eclipse

Summary: American astronomer Joel Stebbins observed Linné Crater during the Feb. 8, 1906, lunar eclipse, which offered visibility to all 50 U.S. states.

The filar micrometer was attached to the University of Illinois Astronomical Observatory's 12-inch equatorial telescope for observations of Linné Crater in February and March 1906: Friends of the University of Illinois Observatory @U.of.Illinois.Observatory, via Facebook Feb. 6, 2020

American astronomer Joel Stebbins observed Linné Crater during the Feb. 8, 1906, lunar eclipse, whose visibility path included all 50 states of the United States of America.

Joel Stebbins (July 30, 1878-March 16, 1966) made his observations at the University of Illinois-Urbana Astronomical Observatory, where he had been serving as second director since 1903. Stebbins reported his observations in the July 18, 1906, issue of the Astronomical Journal.

Stebbins observed the near side crater with a 12-inch equatorial telescope. A filar micrometer was attached to the telescope in order to allow for astrometry measurements. The specialized eyepiece's two wires allowed for precise distance measurements.

His measuring strategy followed the technique espoused by American astronomer and astrophotographer William Henry Pickering (Feb. 15, 1858-Jan. 16, 1938). After placing both wires on the same side of the target, the space between the wires was increased until the width between the wires appeared to equal the target's diameter. He checked the technique with preliminary measurements on the nights of Sunday, Feb. 4, and Monday, Feb. 5.

Measurements were finalized with a correction for the width of a micrometer wire and with rounding. Each observed diameter was reduced by 0".48. Measurements were rounded to one decimal point to avoid the "large systematic errors" inherent in measurements of such a "hazy" object (page 88).

Measuring a small, nearby crater served as a further check on Stebbins' work. The selected check crater lay about 49 degrees northwest of Linné Crater. He approximated its latitude and longitude at 30.03 degrees north, 13.75 degrees west, respectively. In his report, he referred to the check crater as Crater 2.

Stebbins made his measurements of Linné Crater and Crater 2 in a north-south direction. Thus, the direction of measurement was at right angles to the moon's diurnal motion. Earth's eastward (west to east) rotation explains the apparent daily westward (east to west) motion of celestial bodies across Earth's skies.

Stebbins tabulated measurements from four dates. The first date, Monday, Feb. 5, was a preliminary check date. Thursday, Feb. 8, was the date of the total lunar eclipse. Saturday, Feb. 10, and Friday, March 9, provided data for subsequent dates that were distanced from the eclipse by a range of approximately two days to one month, respectively.

He rated seeing on a scale of 0 to 5. Five designated the best seeing. Zero seeing was assigned to Monday, Feb. 5. Seeing at level 2 was experienced prior to the eclipse on Thursday, Feb. 8, and also on both subsequent dates of Saturday, Feb. 10, and Friday, March 9. Seeing at level 1 occurred on Feb. 8 after the eclipse.

Linné Crater's observed diameters ranged from 2.1 to 3.4. Four observations on test date Feb. 5 yielded diameters of 3.0, 3.0, 3.2 and 3.4. The test date's mean diameter was 3.2. No diameters were given for Crater 2.

On Feb. 8, six diameters were secured for Linné between 18:52 Greenwich Mean Time (6:52 p.m. GMT; 12:52 p.m. Central Standard Time) and 19:29 GMT (7:29 p.m. GMT; 1:29 p.m. CST): 2.4, 2.5, 2.5, 2.4, 2.3, 2.4). The Feb. 8 mean diameter was 2.4.

The table did not include measurements for the period after 19:29 GMT (7:29 p.m. GMT; 1:29 p.m. CST) and before 21:19 GMT (9:19 p.m. GMT; 3:19 p.m. CST). As noted by Stebbins, Linné was eclipsed for almost two hours during the event.

A set of five diameters was obtained for Linné between 21:19 GMT (7:29 p.m. GMT; 1:29 p.m. CST) and 21:36 (7:36 p.m. GMT; 1:36 p.m. CST): 3.4, 3.0, 3.2, 3.2, 3.0, 2.6). This set's mean diameter was 3.2.

Stebbins had felt that he was making measurements "as well as possible, and without prejudice" (page 88). At 21:40 GMT (9:40 p.m. GMT; 1:40 p.m. CST), however, a comparison of pre-eclipse settings revealed an enlargement of about 30 percent in measurements of Linné. An immediate decrease in diameter occurred with his next measure, 2.6, at 21:49 GMT (9:49 p.m. GMT; 1:49 p.m. CST).

The bad seeing and the apparent prejudice in his measurements convinced Stebbins to discontinue observations on Feb. 8. He waited almost two days to resume observations in order to forget his eclipse data.

On Saturday, Feb. 10, Stebbins resumed observations. Four diameters were obtained between 16:35 GMT (4:35 p.m. GMT; 10:35 a.m. CST) and 16:47 (4:47 p.m. GMT; 10:47 a.m. CST): 2.6, 2.7, 2.5, 2.6. The date's mean diameter was 2.6.

Approximately one month later, on Friday, March 9, Stebbins revisited Linné. Between 14:41 GMT (2:41 a.m. GMT; 8:41 a.m. CST) and 15:00 GMT (3:00 p.m. GMT; 9:00 a.m. CST), the lunar impact crater's diameters measured 2.2, 2.1, 2.2, 2.1). The date's mean diameter was 2.2.

Stebbins reported that poor seeing seemed to yield larger measurements than those obtained by good seeing. He attributed the post-eclipse large measurements on Feb. 8, which were made at seeing level 1, to this factor.

Contrastingly, Crater 2 diameters did not display a great increase between pre- and post-eclipse measurements on Feb. 8. Diameters were measured pre-eclipse at 2.1, 2.0, 2.1, 2.1 and post-eclipse at 2.4, 2.2, 2.5. The mean diameter was 2.1 pre-eclipse and 2.4 post-eclipse. Linné Crater's "hazy outline" versus Crater 2's well-defined roundness explained the difficulties with determining sizes for Linné.

The takeaways for Joel Stebbins' observations of Linné Crater during the Feb. 8, 1906, lunar eclipse are that the University of Illinois-Urbana Astronomical Observatory's second director encountered difficulties in measuring the near side crater because of poor seeing levels and because of the crater's "hazy outline."

image of Linné Crater obtained July 31, 1971, by Apollo 15 during revolution (orbit) 16, at spacecraft altitude of 103.03 kilometers; NASA ID AS15-P-9348: Public Domain, via NASA

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

The filar micrometer was attached to the University of Illinois Astronomical Observatory's 12-inch equatorial telescope for observations of Linné Crater in February and March 1906: Friends of the University of Illinois Observatory @U.of.Illinois.Observatory, via Facebook Feb. 6, 2020, @ https://www.facebook.com/U.of.Illinois.Observatory/photos/a.137983869590423/2660924930629625/

image of Linné Crater obtained July 31, 1971, by Apollo 15 during revolution (orbit) 16, at spacecraft altitude of 103.03 kilometers; NASA ID AS15-P-9348: Public Domain, via NASA @ https://www.hq.nasa.gov/office/pao/History/SP-362/hrp111.jpg

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