Apparent (astronomy)
The technical-language attribute apparently in astronomy has a meaning that differs from the common language usage. Apparently in the astronomical sense there is no doubt about the reality or reliability of a observed quantity. The technical term describes the observable features of an astronomical object, in contrast to purely computationally determined quantities (e.g. mean positions). This is also expressed in the fact that apparently (in the astronomical sense) is sometimes used synonymously with true (in the astronomical sense).
Apparent sizes are always variables that the observer appear that that is the actual observation and measurement are accessible, though not always immediately. The English word "apparent" , apparently, apparently, according to the impression 'reflects this fact better. As a rule, it is precisely specified how the apparent quantities differ from other quantities, but the exact meaning varies depending on the context.
Apparently in an astronomical sense
Star positions
The position of a star measured by an earthbound observer does not only depend on the location of the star itself, but also on time-dependent changes in the coordinate systems used and various physical environmental influences. Star catalogs can therefore only specify the center position of a star; this must be converted by the user into the apparent position to be actually observed, depending on the observation situation.
The mean position is the position of a star on the celestial sphere as it would be seen by an observer in the center of gravity of the solar system , based on the ecliptic and mean equinox of the date.
The apparent position is the position of the star as it would be seen by an observer in the center of the earth , or as seen from the surface, in relation to the current position of the equator, ecliptic and equinox. To determine the geocentric apparent from the middle position, the proper motion of the star, the precession , the nutation , the annual aberration , the annual parallax and the light deflection in the gravitational field of the sun must be taken into account, with high accuracy requirements additional effects, for the topocentric, for example the daily aberration and parallax and atmospheric light deflection.
The true position of a star is different from its apparent position due to the immense distance and the time that passes at the finite speed of light until its light reaches us . These are recorded in star catalogs as their own movement and must be taken into account in calculations that extend over longer periods of time (around millennia).
Positions of other celestial bodies
While the movements of the heavenly bodies in an ideal two-body system to Kepler tracks follow, is in reality the situation as a multi-body problem by the occurring perturbations much more complex. Therefore is for Ephemeridenrechnungen of celestial mechanics one of computational reasons mean object defined, following a more even course. To calculate the true position, additional terms from the perturbation calculation are then generally used.
To determine the apparent position of a planet - or another object in the solar system - from its geometric position, the time of flight must also be taken into account.
Solar time
The solar time (also: true local time ) is the hour angle of the sun . Because of the ellipticity of the earth's orbit and the inclination of the earth's axis , the hour angle of the sun does not increase strictly uniformly (see equation of time ).
If you determine the solar time by observation from the position of the true (also: apparent ) sun, you get the true (also: apparent ) solar time. Because of the equation of time, it is not strictly uniform.
If you remove the influence of the equation of time mathematically, you get the position of the fictitious so-called mean sun. Your hour angle is the mean solar time .
The sundial measures the “true” local time, i.e. the actual ray of the sun, the mean solar time was a first theoretical approximation to a uniform time calculation, in which at least the days are largely the same throughout the year.
Sidereal time
The sidereal time is the hour angle of the vernal equinox . The position of the vernal equinox in relation to the fixed stars is subject to the precession movement and, superimposed on this, a slight nutation movement .
If you determine the sidereal time from the position of the true (also: apparent ) spring point, that is, taking into account the nutation, you get the true (also: apparent ) sidereal time.
For some purposes it is sufficient to ignore the influence of nutation and only to consider its precession movement to determine the position of the vernal equinox with respect to the fixed stars. The spring point, which is determined in this way and only exists mathematically, is the so-called mean spring point, its hour angle is the mean sidereal time.
The apparent vernal equinox is not a real object and is therefore just as impossible to observe as the mean vernal equinox. However, its position follows directly from observing the movements of the sun and planets.
brightness
The apparent brightness of a star is the brightness measured by an observer (with or without the influence of the atmosphere). In addition to the star's luminosity, it depends primarily on its distance and, if necessary, on the absorption capacity of the interstellar medium between the observer and the star.
To see the actual luminosities of stars compared to each other, one counts on the absolute magnitude of which is the apparent magnitude that would have the star when it at a distance of ten parsecs would.
Apparent size
The apparent size of an object is the angular extent at which it appears to an observer. The sun and moon are roughly the same apparent size, roughly half a degree each . The ring nebula in the lyre has an apparent diameter of about 118 arc seconds .
Apparently in the colloquial sense
Celestial sphere
The distances of the celestial bodies observed range from a few hundred kilometers to several billion light years . For numerous purposes (for example, for tasks in spherical astronomy , for measuring and calculating star positions, etc.) it is sufficient, however, to ignore the different distances and to treat the objects in question as if they were all imagined on the inside of an infinitely large Celestial sphere are attached. In order to clarify its purely imaginary character, one occasionally speaks explicitly of the "apparent celestial sphere".
Rotation of the sky
The earth rotates around itself once from west to east during a sidereal day . For an earthbound observer who believes himself and the earth are at rest, however, the celestial sphere appears to rotate once 360 ° from east to west during this period. Since it is easier for numerous purposes (e.g. coordinate conversions ) to view the celestial sphere and not the observer as moving, in these cases, contrary to all physical knowledge, one often speaks of the "celestial rotation". To make it clear that it is not a physically real movement of the sky, it is sometimes expressly referred to as an "apparent rotation of the sky".
The so-called "apparent daily movement" of the fixed stars is the immediately visible result of the apparent rotation of the sky: It takes place with one rotation within a sidereal day of about 23 hours and 56 minutes, and along the parallel circles of the celestial sphere.
To determine the time from the position of the starry sky, there is the method of the " sky clock ", which gives the respective approximate zone time from the position of the big dipper and the date .
The sun moves with respect to the fixed stars (see next section), by about one degree every day and in the direction opposite to the apparent rotation of the sky. The "apparent daily movement" of the sun caused by the apparent rotation of the sky is therefore somewhat slower than that of the fixed stars; it takes on average a solar day of 24 hours to complete an apparent rotation.
Sun path
While the earth orbits the sun in the course of a year, the sun appears to an earthly observer every day in front of a different fixed star background (at least if one imagines the stars to be visible during the day; with telescopes brighter stars can be observed in the daytime). Once a year the sun seems to move on a great circle (the ecliptic ) around the fixed star sky. It is often more convenient to see the sun as moving and to describe its supposed movement using relevant formulas than to calculate the respective position of the earth and use this to determine the position of the sun to be observed in the fixed star sky. In order to recognize that it is actually the earth that is moving, one occasionally speaks explicitly of the "apparent annual orbit" of the sun.
The same applies to the daily course of the sun over the celestial vault caused by the rotation of the earth . Since the observer believes he can immediately see how the sun moves through its diurnal arc in the daily rhythm , it is usually easier to describe the change in the sun's position as the movement of the sun and not as a mere change in the direction of observation due to the rotation of the earth. If it should be emphasized that it is not a real movement, one speaks explicitly of the "apparent daily path of the sun". While the apparent annual solar path extends along the ecliptic, the apparent daily solar path runs along a parallel circle .
See also
- Apparent star locations (coordinates for the current epoch)
- Apparent cosmic rise and fall , e.g. B. True and apparent sunset
- Apparent star clusters
- Measurements on apparent star orbits
- Apparent planetary orbit , annual planetary loop , apparent right and backward movement of objects in the solar system
- Apparently faster than light objects in astronomy
- Apparent field of view of an eyepiece, telescope field of view
- True and apparent wind in seafaring
literature
- H. Karttunen u. a .: Astronomy - An Introduction . Springer, Berlin 1990, ISBN 3-540-52339-1 .
- Jean Meeus : Astronomical Algorithms . 2nd Edition. Willmann-Bell, Richmond 2000, ISBN 0-943396-61-1 .
- A. Schödlbauer: Geodetic Astronomy . De Gruyter, Berlin 2000, ISBN 3-11-015148-0 .
- PK Seidelmann: Explanatory Supplement to the Astronomical Almanac . University Science Books, Sausalito 1992, ISBN 0-935702-68-7 .