Book contents
- Frontmatter
- Contents
- Preface
- Using your personal computer for astronomy
- DEFAULT: default value input routnine & YESNO: ‘Y’ or ‘N’ input routine
- MINSEC: converts between decimal hours/degrees and minutes/seconds form
- JULDAY: calendar date to Julian day number since 1900 January 0.5
- CALDAY: Julian day number since 1900 January 0.5 to calendar date
- TIME: converts between local civil and sidereal times
- EQHOR: converts between equatorial and horizon coordinates
- HRANG: converts between right ascension and hour angle
- OBLIQ: calculates the value of the obliquity of the ecliptic
- NUTAT: finds corrections for nutation in longitude and obliquity
- EQECL: converts between equatorial and ecliptic coordinates
- EQGAL: converts between equatorial and galactic coordinates
- GENCON: converts between any of the coordinate systems
- PRCESS1: approximate precession of equatorial coordinates & PRCESS2: rigorous precession of equatorial coordinates
- PARALLX: converts between geocentric and apparent position
- REFRACT: calculates the effect of atmospheric refraction
- RISET: finds the circumstances of rising and setting
- ANOMALY: solves Kepler's equation for elliptical motion
- SUN: finds the ecliptic coordinates of the Sun
- SUNRS: finds the circumstances of sunrise and sunset
- PELMENT: returns the orbital elements of the major planets
- PLANS: finds the position of a planet
- MOON: finds the position and parallax of the Moon
- MOONRS: finds the circumstances of moonrise and moonset
- MOONNF: finds the times of new and full moon
- ECLIPSE: finds the circumstances of lunar and solar eclipses
- DISPLAY: displays an eclipse in graphical form
- ELOSC: finds positions from osculating elliptical elements
- RELEM: converts elliptic orbital elements from one epoch to another
- PCOMET: finds the position of a comet from parabolic elements
- PFIT: finds parabolic elements from observations & EFIT: finds elliptical elements from observations
- List of variables
- Bibliography
- Index
- PROGRAMS AVAILABLE ON DISK
REFRACT: calculates the effect of atmospheric refraction
Published online by Cambridge University Press: 17 February 2010
- Frontmatter
- Contents
- Preface
- Using your personal computer for astronomy
- DEFAULT: default value input routnine & YESNO: ‘Y’ or ‘N’ input routine
- MINSEC: converts between decimal hours/degrees and minutes/seconds form
- JULDAY: calendar date to Julian day number since 1900 January 0.5
- CALDAY: Julian day number since 1900 January 0.5 to calendar date
- TIME: converts between local civil and sidereal times
- EQHOR: converts between equatorial and horizon coordinates
- HRANG: converts between right ascension and hour angle
- OBLIQ: calculates the value of the obliquity of the ecliptic
- NUTAT: finds corrections for nutation in longitude and obliquity
- EQECL: converts between equatorial and ecliptic coordinates
- EQGAL: converts between equatorial and galactic coordinates
- GENCON: converts between any of the coordinate systems
- PRCESS1: approximate precession of equatorial coordinates & PRCESS2: rigorous precession of equatorial coordinates
- PARALLX: converts between geocentric and apparent position
- REFRACT: calculates the effect of atmospheric refraction
- RISET: finds the circumstances of rising and setting
- ANOMALY: solves Kepler's equation for elliptical motion
- SUN: finds the ecliptic coordinates of the Sun
- SUNRS: finds the circumstances of sunrise and sunset
- PELMENT: returns the orbital elements of the major planets
- PLANS: finds the position of a planet
- MOON: finds the position and parallax of the Moon
- MOONRS: finds the circumstances of moonrise and moonset
- MOONNF: finds the times of new and full moon
- ECLIPSE: finds the circumstances of lunar and solar eclipses
- DISPLAY: displays an eclipse in graphical form
- ELOSC: finds positions from osculating elliptical elements
- RELEM: converts elliptic orbital elements from one epoch to another
- PCOMET: finds the position of a comet from parabolic elements
- PFIT: finds parabolic elements from observations & EFIT: finds elliptical elements from observations
- List of variables
- Bibliography
- Index
- PROGRAMS AVAILABLE ON DISK
Summary
Observations from the Earth's surface have to be made through the atmosphere. Although the air is tenuous, it has a refractive index which is significantly greater than 1, the value for a vacuum. When we observe a celestial object at an angle from the vertical the rays are bent slightly away from the path they would have followed if the atmosphere had not been present. The atmosphere clings to the curve of the Earth and it acts rather like a weak lens, making stars appear to be nearer to the vertical than they really are. The largest effect is seen at rising and setting where the horizontal refraction is about 34 minutes of arc. Since the effect is always to increase the Sun's apparent altitude, the length of the day is increased because of it.
Subroutine REFRACT assumes a model atmosphere whose refractive properties depend only on pressure, PR</b.>, and temperature, TR, at the place of observation. The amount of refraction actually observed depends on the detailed disposition of the atmosphere along the line of sight to the celestial object in question, and significant deviations from the values calculated here must be expected in some instances, especially near the horizon. Nevertheless, REFRACT will provide a very good approximation which will be more than adequate in most circumstances.
The altitude of the object is input via the parameter Y, corrected by the routine, and returned via the parameter Q. The direction of correction is indicated by the switch SW(6).
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- Astronomy with your Personal Computer , pp. 99 - 104Publisher: Cambridge University PressPrint publication year: 1990