Sidereal Time and Hour angle (english version)
Sidereal time
When talking about times or periods in astronomy, and added the sidereal qualifier is to
point that the benchmark is the sky, the stars.
The word "sidereal" comes from the
Greek "
Iron" (σίδερο = Sidero) and applies to the stars because the meteorites fallen from the sky had a large quantity of this metal, and
old attributed the iron to stars.
Well, the sidereal time is the time measured by movement of the
Vernal Point (or
Aries Point) on a place on the surface of the
Earth. The Vernal Point (0 degrees Aries) is the point of intersection between
Celestial Equator and the ecliptic plane.
This point is also known as
Spring Equinox and is one of the two equinoctial points.
The other is its opposite,
Libra point, or autumn equinox.
This point is not really fixed, because it retrograde 52 seconds per year, due to the movement of precession of the equinoxes, but for purposes of the sidereal time measurement s a good benchmark.
When the
Vernal Equinox cuts the local meridian, the sidereal time is 0 hours 0 minutes. In this moment begins the sidereal day. The sidereal day does not last 24 hours, as the solar day, but is slightly shorter, 23 hours and 56 minutes.
This means that every sidereal day is about
4 minutes shorter than the solar day.
Why is this way?
Consider an observer somewhere.
Look at the meridian of that place.
Remember what is the meridian: is a plane that intersecting the vertical, passes both by the north and south cardinal points.
This plane helps us to know when it is noon, because the Sun cut the meridian at this moment. The observer is located in the
Northern Hemisphere and he look toward the
South.
To measure a solar day, we count the time it takes
the sun to pass twice by
that meridian and this period we established by convention that are 24 hours. But the Sun is much closer to us than the stars so the
difference is that when the Earth shift in orbit, the sun has moved from the previous day near a degree in the sky to observer's left, that is towards the
West.
So, when the reference star, or Vernal Point, culminate, that is, when passing through the local meridian, the sun has not yet reached the culmination, and indeed
still need about 4 minutes to do.
The
Sidereal time, then, is the distance in hours, minutes and seconds of the Vernal Point to local meridian. When this point culminates in the local meridian, the sidereal time is 0 hours 0 minutes, as we have said already.
The Hour Angle (H) is the distance in hours, minutes and seconds between a star and the local meridian. When the
Hour Angle is 0 hours 0 minutes the star culminate on the meridian, that is, at the upper meridian.
Fundamental equation in astronomy of position.
The fundamental relationship in astronomy of position
is this:
Local Sidereal Time (
LST) is equal to local hour angle (
LHA)
+
Right Ascension (RA) of an object.
That is, if we add the Right Ascension (RA) of a
star, to Local Hour Angle (LHA). That always depend on our meridian
local always will result in our Local Sidereal Time (LST).
©
Antonio González Estévez
2012 2013 2014
© Música
Nacho Rouco Estévez 2012 2013 2014