As explored in the previous post, asteroid 433 Eros was an area of major study at the turn of the 19th century. The first goal of studying Eros, in 1899, was to generate an accurate ephemeris for the asteroid. This feat was first accomplished by Chandler and Fleming of the Washburn Observatory for Eros in 1901. With a new, more accurate ephemeris, astronomers had the opportunity to study the asteroid during opposition (closest approach to earth) in an attempt refine the Astronomical Unit (mean distance from the Earth to the Sun).
In 1900-1901, a worldwide campaign was launched to measure the parallax of Eros. The Harvard College Observatory (HCO) assisted in announcing the opposition and its implications in measuring the astronomical unit. In this announcement, written by the director of the observatory Edward Pickering, an ephemeris generated by HCO is provided so that those who received the document, and wanted to observe, would know where to look. 
If an astronomer in Cambridge England were to photograph Eros at the same time someone in Cambridge Massachusetts did, they would see that Eros would appear to be in a slightly different position relative to the background stars. This concept is called parallax and can be easily demonstrated. Hold your arm out at shoulder height. Point your thumb up and close one eye. Now close the open eye and open your other eye. You will notice that your thumb appears to change in position relative to whatever is behind it. If you know the distance between your eyes and the angle formed by your thumb and eyes, you can determine the length of your arm. This same method was applied during the Transit of Venus in 1874. As a result, the distance was calculated to be approximately 95,000,000 miles (153000000 kilometers). The results from the Eros study promised to be far more accurate than results that could be achieved from the transit of Venus.
The final reductions to calculate the parallax of Eros was carried by Arthur Hinks at the Cambridge Observatory in England. Hinks released these results in Solar Parallax Papers. No. 7. The parallax angle calculated there was Δπ=+0.007’’±0.0027’’, which equates to about 92,890,000 miles (about 149,000,000 kilometers). This is quite close to the value of the astronomical unit used today which is 92955807.3 miles or 149697871 km. While Eros played a major role in refining one of the standard units in Astronomy, its job as an object to break new ground was not yet finished.
The above picture shows part of the data taking process for the parallax measurements, having multiple observers at different parts of the Earth measuring the position of Eros in the sky relative to background stars.
1. Pickering, Edward Charles. “Opposition of Eros in 1900.” The SAO/NASA Astrophysics Data System, 1900. Web. 22 July http://articles.adsabs.harvard.edu//full/1900HarCi..49….1P/0000001.000.html
2. Hinks, Arthur. “Solar Parallax Papers. No. 7.” The SAO/NASA Astrophysics Data System, n.d. Web. 1 Aug. 2013.
3. Jones, Bessie Zaban, and Lyle Gifford Boyd. The Harvard College Observatory: The First Four Directorships, 1839-1919. Cambridge, MA: Harvard UP, 1971. Print. p345.
4. Simmons, Michael. “Magazine.” Sky and Telescope. N.p., 29 Jan. 2012. Web. 26 Sept. 2013. http://www.skyandtelescope.com/observing/highlights/The-Eros-Parallax-Project-138301789.html