Locate an Object in the Sky

Real Movement of Celestial Objects

Ecliptic plan ] This is the geometric plane containing the orbit of the earth around the Sun. All the orbits of the planets of the Solar System and the Moon are in this plane. Note that Pluto, which since 2006 has lost its planet status to become a dwarf planet has an orbit at an angle to the plane of the ecliptic.

Ecliptic ] From a geocentric perspective, the ecliptic is the projection of the annual path of the sun on the celestial sphere (see “celestial sphere” below below). from a heliocentric point of view, it is the intersection of the celestial sphere with the ecliptic plane (geometric plane containing the orbit of the earth around the sun).

Orbit of the Moon ] The moon makes an orbit around the earth in about 27.3 days, about 385,000 miles from earth on average center at an average speed of 1023 m / s1. it differs from most of the moons of other planets in its orbit closest to the plane of the ecliptic as the earth’s equator.
although generally elliptical the orbit of the moon has many irregularities (known disturbances).

Sidereal Day ] The time it takes a planet to make a turn on itself relative to the stars (360 ° rotation), regardless of its revolution around the sun. . it also corresponds to the time it takes a planet to make a turn on itself with respect to the vernal equinox (see “equatorial coordinates” below).
→ 23 hours, 56 minutes and 4 seconds

Solar Day ] The time it takes a planet to make a turn on itself from the sun (rotation of 361°). From a geocentric point of view, a solar day is the time between two culminations of the sun (eg from noon to noon the next day).
→ 24 hours

While the earth rotates on itself (> rotation), it also revolves around the sun (> revolution), making its orbit in a year. in 24 hours, The earth steering – sun changes by about 1 °. So for a given point of the earth is again facing the sun, it must rotate 361 °. This is why the solar day is four minutes longer than the sidereal day. And that is also why the stars seem slide each day a little more to the west. in addition, the time of rotation and revolution varies planets. on earth, a solar day has almost the same length as a sidereal day, while on mercury, the difference is noticeable: a solar day lasts 176 earth days because of its low speed and rapid orbital speed, then a sidereal day lasts 58.65 earth days (three times less time).

Earth Year ] Time that the earth takes to make one complete revolution.

Axial Tilt of Earth ] The rotation axis of the Earth is not perpendicular to the plane of the ecliptic but inclined at an angle of 23.5 °. That is why the path of the sun in the sky seems higher or lower depending on the season which, by the way, are also the result of this tilt.

Axial Precession ] Very slow conical movement of the rotation axis of the Earth around the perpendicular to the ecliptic plane, due to the overwhelming attraction of the Sun and the Moon on the Earth. It takes about 25,800 years to complete one revolution.

>> Polar Stars Changes : The polar axis that now points to the North Star will point to Alderamin in 8000 AD.

Apparent Motion of Celestial Objects

Apparent Motion ] The movement of celestial objects is only apparent. It is due to the rotation of our planet on itself and its revolution around the sun. The earth’s rotation gives the impression that the stars rise in the east and west lie.

>> The apparent motion of the sky, which is almost imperceptible to the naked eye or with binoculars, a telescope is obvious because it is magnified by the magnification. For example, with a magnification of 200 times, the movement of the observed object is displayed 200 times faster than the naked eye.

Circumpolar Celestial Object ] Object that, from a given location on earth, do not layer below the horizon. For example, in the northern hemisphere, ursa major, ursa minor and some other relatives of the heavenly constellations are circumpolar north pole.

There are two special cases :
► If there is geographic poles, the center of the circular motion of the stars will be located at the zenith. Therefore, all the visible stars keep the same height above the horizon and never will sleep. So in this case, all visible stars are circumpolar.
► However, if one is to the equator, all the stars rise and set in the west. There is then no circumpolar star.
Therefore, the closer you get the geographic poles, and the sky is dotted with circumpolar stars.


Unit used to measure the apparent distance between two points of the sky, or the apparent diameter of a celestial object. One degree is divided into 60 arcminutes (symbol ‘, prime), further divided into 60 seconds of arc (symbol ‘ ‘, double prime).
1 ‘ (arcmin) = 1/60 = 0.016 666°
1 ‘ ‘ (arcsec) = 1/3600 = 0.000 277°

Celestial Sphere

The celestial sphere is an imaginary globe surrounding the earth, which has the observer’s eye center, on which all visible astronomical objects are projected. It allows you to locate celestial objects by assigning their coordinates, called celestial coordinates, and so, to identify more quickly. Compared to the distances that separate us from the stars, the radius of the earth can be considered mathematically and therefore as the center of the sphere. Moreover, its radius is indeterminate, as seen from the observer, the distance of the stars in relation to it has no influence over their apparent positions in the sky. The visible part of the celestial sphere, that is to say the hemisphere overlooking the observer is called sky.

Celestial Equator ] This is the projection of the earth’s equator onto the celestial sphere.

Celestial Poles ] These are the two points of intersection, north and south, the axis of rotation of the earth and the celestial sphere. These are the pivots around which the entire sky appears to rotate (in exactly 23h56’04 = sidereal day).

Polaris ] Term for a star visible to the naked eye and located at the site of one of the two celestial poles. as precession varies the positions of the poles in a period of about 25 800 years, the pole stars are caused to vary. Currently the north polar star is α ursae minoris or polaris. It belongs to the constellation ursa minor. The southern hemisphere has no polar star, the closest being sigma octantis located 1 ° south celestial pole and having a low light.

Vernal Point ] The celestial equator and the ecliptic intersect at two points called nodes. When the sun passes from the northern hemisphere to the southern hemisphere, it crosses the descending node. It passes the southern hemisphere to the northern hemisphere, it crosses the ascending node, called the vernal point (or point of the vernal equinox or point of the spring equinox). It is located near the constellation of pisces. The right ascension line zero, or zero meridian passes through this point vernal.

Celestial Coordinates

There are two types of celestial coordinates:


The equatorial coordinate system used to locate the position of a star regardless of time and place. To put a point on our globe, using two coordinates: latitude and longitude. Latitude defines the distance from a point north or south from the equator, while longitude defines the distance east or west from the main meridian, the meridian greenwich (passing through london). when projected lines of latitude and longitude on the celestial sphere, the equatorial sky coordinates we get :

► Latitude is then called declination (dec). The celestial equator has a declination of 0 °. The variations are then rated in positive degrees between the celestial equator and the north pole (90°) and negative degrees to the south pole (- 90 °).
► The longitude is called right ascension (ad). All known celestial objects are located through these two coordinates. Right ascension is measured in hours and not in degrees, an hour on the celestial sphere equal to 15 ° terrestrial longitude. There are a total of 24 hours.

>> A telescope equipped with an equatorial mount allows you to locate stars which we know the equatorial coordinates with a dial to the right ascension, and one for the declination.


They are also called alt-azimuth coordinates local celestial or horizontal. This time, the coordinates of celestial objects will depend on the time and place of observation.
The azimuthal system is defined by two coordinates :

► Altitude or height, which reference the local horizon. It is the angle between the target star and the horizon (or horizontal). In degrees, minutes and seconds, from 0° to 90° to the zenith and from 0° to -90° to the nadir. The horizon therefore at an altitude of 0°.
► The azimuth, which reference the geographic north. It is the horizontal angle between true north and the vertical projection of the sun over the horizon. The azimuth is counted in degrees, minutes and seconds (from 0° to 360°) in the backward direction, that is to say, clockwise around the table. northern therefore an azimuth of 0°, 90° east, south 180 ° and 270° west.

>> A telescope equipped with an azimuthal mount allows you to locate stars which we know azimuthal coordinates.