Datasets:
vnpthack
/
vnpt_data

Dataset card Data Studio Files
xet
 Community
1
text
stringlengths
0
6.73k
carth has been taken as the basic standard of time. As measurements have been
made more and more precise, however, it has been found that the rotation of the
earth is not exactly periodic, when measured ïn terms of the best clocks. 'These
“best” clocks are those which we have reason to believe are accurate because they
agree with each other. We now believe that, for various reasons, some days are
longer than others, some days are shorter, and on the average the period of the
earth becomes a little longer as the centuries pass.
--- Trang 109 ---
Until very recently we had found nothing much better than the earth's period,
so all clocks have been related to the length of the day, and the second has been
defned as 1/86,400 of an average day. Recently we have been gaining experience
with some natural oscillators which we now believe would provide a more constant
time reference than the earth, and which are also based on a natural phenomenon
available to everyone. 'These are the so-called “atomic clocks.” 'Their basic internal
period is that of an atomiec vibration which is very insensitive to the 6emperature
or any other external efects. Thhese clocks keep time to an accuracy of one part
in 102 or better. Within the past two years an improved atomic clock which
operates on the vibration of the hydrogen atom has been designed and built by
Professor Norman Ramsey at Harvard University. He believes that this clock
might be 100 times more accurate still. Measurements now in progress will show
whether this is true or not.
We may expect that since it has been possible to build clocks mụuch more
accurate than astronomical time, there will soon be an agreement among scientists
to defñne the unit of tỉme in terms of one oŸ the atomiec clock standards.
5-6 Large distances
Let us now turn to the question oŸ đjs‡ønece. How far, or how bịg, are things?
lverybody knows that the way you measure distance is to start with a stick and
count. Or start with a thumb and count. You begin with a unit and count. How
does one measure smaller things? How does one subdivide distance? In the same
way that we subdivided time: we take a smaller unit and count the number of
such units it takes to make up the longer unit. So we can measure smaller and
smaller lengths.
But we do not always mean by distance what one gets by counting of with
a meter stick. It would be difcult to measure the horizontal distance between
two mountain tops using only a meter stick. We have found by experience that
distance can be measured in another fashion: by triangulation. Althouph this
mmeans that we are really using a diferent definition of distance, when they can
both be used they agree with each other. Space 1s more or less what Euclid
thought it was, so the two types of defnitions of distance agree. Since they do
agree on the earth it gïves us some confdence in using triangulation for still larger
distances. Eor example, we were able to use triangulation to measure the height
of the first Sputnik. We found that it was roughly 5 x 10” meters high. By more
careful measurements the distance to the moon can be measured in the same
--- Trang 110 ---
V CÀ ccccccceeccceeefng
ễP6Ệ555=Eœ
Fig. 5-4. The height of a Sputnik ¡is determined by triangulation.
way. wo telescopes at diferent places on the earth can give us the two angles
we need. It has been found in this way that the moon is 4 x 10 meters away.
W© cannot do the same with the sun, or at least no one has been able to yet.
"The accuracy with which one can fÍocus on a given point on the sun and with which
one can measure angles is not good enough to permit us to measure the distance
to the sun. 'PThen how can we measure the distance to the sun? We must invent an
extension of the idea of triangulation. We measure the relative distances of all the
planets by astronomical observations of where the planets appear to be, and we
get a picture of the solar system with the proper relate distances of everything,
but with no absolu£e distance. Ône absolute measurement is then required, which
has been obtained in a number of ways. One of the ways, which was believed
until recently to be the most accurate, was to measure the distance from the
earth to Eros, one of the small planetoids which passes near the earth every now
and then. By triangulation on this little object, one could get the one required
scale measurement. Knowing the relative distances of the rest, we can then tell
the distance, for example, from the earth to the sun, or tom the earth to Pluto.
'Withim the past year there has been a big improvement in our knowledge of
the scale of the solar system. At the Jet Propulsion Laboratory the distance from
the earth to Venus was measured quite accurately by a direct radar observation.
'This, of course, is a still diferent type of inferred distance. We say we know the
specd at which light travels (and therefore, at which radar waves travel), and we
assume that ï is the same speed everywhere between the earth and Venus. We
send the radio wave out, and count the time until the relected wave comes back.
trom the #ữne we infer a đis‡ønce, assuming we know the speed. We have really
another defñnition of a measurement of distance.
How do we measure the distance to a star, which is much farther away?
tFortunately, we can go back to our triangulation method, because the earth
--- Trang 111 ---
ASTAR
TT T~`
⁄ SUN N
ÁSE1fssmsx_)annanEAslồ
^ ¬ - — ~Z ⁄
Fig. 5-5. The distance of nearby stars can be measured by triangula-
tion, using the diameter of the earth's orbit as a baseline.
moving around the sun gives us a large baseline for measurements of objecEs
outside the solar system. lÝ we focus a telescope on a star in summer and in
winter, we might hope to determine these two angles accurately enough to be
able to measure the distance to a star.
'What ïf the stars are too far away for us to use triangulation? Astronomers
are always inventing new ways of measuring distance. They fnd, for example,
that they can estimate the size and brightness of a star by its color. The color and
brightness of many nearby stars—whose distances are known by triangulation——
have been measured, and ït is found that there is a smooth relationship between the
color and the intrinsic brightness of stars (¡in most cases). IÝone now measures the
color ofa distant star, one may use the color-brightness relationship to determine
the intrinsic brightness of the star. By measuring how bright the star øppears to
us at the earth (or perhaps we should say how đớn it appears), we can compute
how far away it is. (Eor a given intrinsic brightness, the apparent brightness
decreases with the square of the distance.) A nice confirmation of the correctness
of this method of measuring stellar distances is given by the results obtained for
groups of stars known as globular clusters. A photograph of such a group is shown
in Eig. 5-6. Just from looking at the photograph one is convinced that these