Sunday, July 24, 2011

An Ancient Egyptian Leveling Tool

In Ancient Egypt an A-shaped wooden frame with a plumb bob was used as a leveling tool. It was known as a khekh. An example of this is the triangle level found in the tomb of Sennedjem from the 19th dynasty.

Source: Budge, An Egyptian Hieroglyphic Dictionary

The Bouguer Anomaly

The Bouguer Anomaly is a contribution to the deviation from vertical caused by irregularities in the Earth's gravitional field due to variations in density. It is most noticable near mountainous regions where it can be on the order of one minute of arc. Changes in altitude also result in gravity anomalies since gravitation is a function the distance from the Earth's center. Gravity anomalies have been known for a long time.The Schiehhallion Experiment of 1774 which involved the gravitational deflection caused by a mountain in Scottland which was used in an attempt to measure the mean density of the Earth and hence the gravitational constant.

Monday, July 18, 2011

Galileo Satellite Navigation System

The European Galileo satellite navigation system is about to enter a new phase that of in-orbit validation (IOV). The first IOV satellites are due to be launched within the next year and will enable ESA to test the satellite and ground station systems before the entire constellation of satellites is placed in orbit. This is the topic of a recently released ESA video.

Monday, July 11, 2011

ERS-2 Images of the Kangerdlugssuaq Ice Stream

ESA has recently posted ERS-2 images of the Kangerdlugssuaq ice stream in Greenland which is advancing at 35 meters per day.

Credits: ESA

There are claims that this is due to global warming but one should also note that Greenland has many hot springs and these may be a contributing factor. This part of Greenland faces Iceland which has experienced some recent volcanic eruptions. Global warming produces the greatest temperature increase in the Artic which can be seen by the decline of the northern ice pack. Climate changes slowly over time and a retreat of the glaciers has been taking place over the 10,000 years since the last glacial period when much of the northern hemisphere was covered by ice sheets.

Supplemental readings: Glacial meltwater, Subglacial lake, Jökulhlaup, Meltwater pulse 1A, Tunnel valley, fjord, glacial valley. (Is history repeating itself? One gets the impression that ice sheets and glaciers have rather complicated mechanisms associated with them.)

Sunday, July 3, 2011

Earth at Aphelion on the 4th of July

MICA, the US Naval Observatory's Multiyear Interactive Computer Almanac, indicates that the Earth will be at aphelion, the furthest distance from the Sun, on 4 July 2011 at about 14:54 UT.

The terms perihelion and aphelion were first used in connection with the epicycle discription of the motion of the Sun, Moon and planets referring to points in their orbits so these concepts may be over 2000 years old. Ptolemy uses the terms "perigee" and "apogee" since his epicycle orbits were centered on the Earth. The composite motion of epicycle and deferent is somewhat elliptical and are a good approximation to the motion of the Sun and Moon so it is not surprising that there are near and far points in these "orbits." Ptolemy gives Gemini 5° 30' or 65° 30' for the position of the Sun's apogee.

Supplemental: When comparing Kepler's elliptical orbits with Ptolemy's epicycles it might be fairer to think in terms of the relative motion of the Sun and the Earth. According to Archimedes, Aristarchus postulated that the Earth circled the Sun circa 300 BC. Ptolemy may have been aware of this since he had access to Archimedes (see Almagest p. 44). It is probably best to separate observations from theory as much as possibly to avoid contaminating them with errors unnecessarily. Especially when the observations are solely Earth based.