astronomical survey

7/21/2019 Astronomical Survey

http://slidepdf.com/reader/full/astronomical-survey 1/15

ASTRONOMICAL SURVEY



TIME, APPARENT TIME,

AND SIDEREAL TIME



DEFINITION OF TIME:

• the indefinite continued progress of existence a

events in the past, present, and future regarde

as a whole.

•

It is also considered as a point of time asmeasured in hours and minutes past midnight

noon.



DIFFERENT SYSTEMS USED IN THE

DETERMINATION OF TIME:

1.Apparent Solar Time

2.Sidereal Time

3.Mean Solar Time4.Standard Time



APPARENT TIME

• This method of time determination is resolved withrespect to the apparent sun or the real sun.

• This method employs the operation of a sun dial.

• The time required for an apparent revolution of the

earth around the sun is called as a solar day.• It begins at when the true sun starts to appear on th

lower branch of the observer’s meridian.

• It is also referred to as sun time.



• However, the sun is not a good indicator because the

length of the apparent solar day is not constant. Due

to the following reasons:

–The non-constant velocity of the apparent sun on

the elliptical path as it moves across the sky.

–The amount of time that light is required to travel

on varying points on the elliptical orbit of the

Earth.

–Equinoxes and Solstices



SIDEREAL TIME

•

A sidereal day at any place on the earth’s surface begiwhen the vernal equinox is on the observer’s meridian

and above the horizon.

• This means that after 24 hours the vernal equinox is

again at meridian passage and 1 sidereal day passes.

• It is considered as a star time.

• An apparent solar day is 3 minutes and 56 seconds

longer than a sidereal day which is 23 hours and 56.1

minutes



• At any point on the earth’s surface, the sidere

time is equivalent to the hour angle of the

vernal equinox referred to as the meridian ofthe place.

• Sidereal time is measured by special clocks

which are regulated so as to gain 24 hours a

year over ordinarily used time pieces.



DIFFERENCE IN SOLAR TIME AND SIDEREAL T

At time 1, the Sun and a cer

distant star are both overheadtime 2, the planet has rotated

and the distant star is overh

again (1→2 = one sidereal day).

it is not until a little later, at tim

that the Sun is overhead again (

= one solar day). More simply, 1

a complete rotation of the Earth,

because the revolution around

Sun affects the angle at which

Sun is seen from the Earth, 1-

how long it takes noon to return.



This photo album contains samp

pages to get you started.

To add your own pages, click the

Home tab, then click the New Sl

gallery.



…then click the placeholders to add your own pictures and captions.

CHOOSE A LAYOUT



On the Picture Tools | Format t

you can create your own frames

make picture corrections such a

adjusting contrast and brightne

cropping the picture for just thelook.



Picture Quick Styles give you great looking “frames” in a single click.

astronomical survey

Documents