11/4/2023 0 Comments Right ascension definition 427Tropical year = length of time between Vernal equinoxes Sidereal year = length of time for the Earth to return to the same position with respect to the stars Note that several different lengths of a year can be defined depending on what reference is used: We also use the time that it takes the Earth to orbit the Sun as a time unit, the year. These differences result from two causes - first, we measure time with respect to the Earth's rotation axis and hence equator while the Sun's motion is along the ecliptic, and second, the Earth's orbit is elliptical and the Earth moves faster along its orbit when closer to the Sun. noon), you will discover a complicated pattern with the differences between these intervals and an average day called the equation of time. If you keep track of the intervals between successive times of maximum elevation of the Sun above the horizon (e.g. The Earth's rotation is used a basic unit of time measure - the day. For example, the elevation of the North Star above an observer's horizon is equal to the observer's latitude: Positions of stars and other objects in the sky can be used for navigation (now laregely made obsolete by Global Positioning Systems). Spherical trigonometry is handy for converting between coordinates systems. Coordinates based on a telescope's location can be advantageous as can coordinates based on the plane of the Solar System or on the plane of the Milky Way galaxy. In addition to RA and DEC, astronomers use a number of other coordinate systems depending on the circumstances. This illustrates how the equatorial system accounts for the rotation of the Earth and explains why early astronomers chose to use time units for right ascension. for an object is negative, it lies east of the meridian and will cross the meridian, reaching its highest point in the sky, when the sidereal time equals its right ascension. It is related to time as measured by the stars called sidereal time via the relation The hour angle is the distance in time units that object lies east or west of the meridian. or hourĪngle of an object is also useful to know. When using equatorial coordinates, the H.A. It is very important to keep minutes of time and arc minutes clear!Įxamples: A star on the celestial equator with right ascension 6 hrs lies 6 hrs x 15 deg/hr = 90 degrees from the Vernal equinox.Ī star at 60 deg declination and right ascension 6 hrs lies 6hrs x 15 deg/hr x cos(60) = 45 degrees from a point at 60 deg declination and 0 hrs right ascension. Another complication with right ascension arises from the changing angular size of circles of constant right ascension when moving from the celestial equator towards the celestial pole, the circles shrink by a factor of cos(DEC) which must be taken into account.ġ' = 1/15 min = 60/15 secs = 4 secs 15' = 1 minute Note that because right ascension is measured in time units, before performing calculations you need to multiply by 15 degrees/hr. Declination is usually expressed in degrees, minutes of arc, and seconds of arc.ġ degree = 1 º = 60 arc minutes = 60' = 3600 arc seconds = 3600" The 15 deg/hr conversion factor arises from the rotation rate of the Earth.ĭeclination is analogous to latitude and is measured as north or south of the celestial equator. The total range of right ascension is 24 hrs = 360 deg / 15 deg/hr. The zero-point for right ascension is the Vernal Equinox (also called the Aries Point in the text), location on the celestial equator of sunrise on the first day of spring. Right ascension is analogous to longitude, is usually measured in units of time: hours, minutes, seconds. This system was the first used in compiling stellar catalogues, and the two coordinates used to define the location of an object, right ascension (abbreviated often as or RA ) and declination ( or DEC), are in common use today. Several different coordinate systems are useful depending on the situation:Įquatorial coordinates: An Earth-based system useful for pointing telescopes with axes that are parallel to the Earth's polar axis and equator (called equatorial mount telescopes). The pointĭirectly overhead is called the zenith (and the point 180º away is called the nadir). The meridian is the circle runningįrom one pole to the other through a point directly overhead for an observer. The celestial poles and equator are the projections of the Earth's poles and equator onto the sky. On the concept of the celestial sphere upon which all objects are assumed to lie regardless of their true distances. Astronomers base their measurement of positions of objects
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