19 (1) Flashcards
One of the enduring legacies of the era of the French emperor Napoleon is the establishment of the metric system of units, officially adopted in France in ……………. and now used in most countries around the world.
1799
An intermediate standard meter consisting of a bar of ……………………. metal was set up in Paris. In ……………, by international agreement, this bar was defined to be exactly one meter in length, and precise copies of the original meter bar were made to serve as standards for other nations.
platinum-iridium / 1889
In 1960, the official definition of the meter was changed again. As a result of improved technology for generating spectral lines of precisely known wavelengths, the meter was redefined to equal 1,650,763.73 wavelengths of a particular atomic transition in the element ……………
krypton-86
The advantage of this redefinition is that anyone with a suitably equipped laboratory can reproduce a standard meter, without reference to any particular metal bar.
The meter was redefined by international agreement in ………….. as the length of the path traveled by light in a vacuum in 1/299,792,458 of a second.
This definition also locked the speed of light at 299,792,458 meters per second in a vacuum.
1983
We could just as well use the light-second as the fundamental unit of length, but for practical reasons (and to respect tradition), we have defined the meter as a small fraction of the light-second.
What is a light second?
It is defined as the distance that light travels in free space in one second
There is also a light minute/hour and a year
The work of Copernicus and Kepler established the …………… distances of the planets—that is, how far from the Sun one planet is compared to another. But their work could not establish the ……….. distances (in light-seconds or meters or other standard units of length).
- relative / absolute*
- This is like knowing the height of all the students in your class only as compared to the height of your astronomy instructor, but not in inches or centimeters. Somebody’s height has to be measured directly.*
The key to our modern determination of solar system dimensions is …………., a type of radio wave that can bounce off solid objects. By timing how long a …………. beam (traveling at the speed of light) takes to reach another world and return, we can measure the distance involved very accurately.
radar
In …………, radar signals were bounced off …………. for the first time, providing a direct measurement of the distance from Earth to ………. in terms of ……………
1961 / Venus / Venus / light-seconds
Subsequently, radar has been used to determine the distances to …………, ……………. the satellites of………….., the rings of ……………, and several asteroids.
Mercury, Mars / Jupiter / Saturn
Can I use the Radar to measure the distance to the sun? Why? why not?
it is not possible to use radar to measure the distance to the Sun directly because the Sun does not reflect radar very efficiently.
Since not possible to use radar to measure the distance to the Sun directly, we can measure the distance to many other solar system objects and use ………….’s laws to give us the distance to the Sun.
Kepler
When Earth and the Sun are closest, they are about ……….. …………. kilometers apart; when Earth and the Sun are farthest, they are about ………. ……….. kilometers apart.
The average of these two distances is called the astronomical unit (AU).
147.1 million / 152.1 million
The length of 1 AU can be expressed in light travel time as …………. light-seconds, or about ……….. light-minutes.
499.004854 / 8.3
What is the speed of light in Meters/s?
c = 3 * 10^8 m/s
300,000,000 m/s / Three hundred million meter in a second
What is the speed of light in Kilometers/s?
c = 3 * 10^5 km/s
300,000 Km/s / Three hundred thousand Kilometers in a second
we can, in principle, survey distances to the stars using the same technique that a civil engineer employs to survey the distance to an inaccessible mountain or tree—the method of ……………….
triangulation
The ………… is also the angle that lines AC and BC make—in mathematical terms, the angle subtended by the baseline. A knowledge of the angles at A and B and the length of the baseline, AB, allows the triangle ABC to be solved for any of its dimensions—say, the distance AC or BC. The solution could be reached by constructing a scale drawing or by using trigonometry to make a numerical calculation.
parallax
