Meteorologically speaking and calendrically speaking spring began on the first of March, and the weather in Scotland has certainly been brighter this month after a rather dull and damp February. In this month's article we will look at 'astronomical spring'; the spring equinox and how it was used as the basis for the (right ascension/declination) celestial coordinate system.
We also take a very brief glimpse into the life of the ancient Greek astronomer Hipparchus and the Hipparcos astrometry satellite named in his honour. We briefly look at the International Celestial Reference System which is now used in professional astrometry in place of the traditional RA/Dec system.
Du point de vue météorologique le printemps commence le premier mars, et heureusement ce mois le temps a été assez clément et printanier en Ecosse après un février plutôt gris et humide.
Aujourd’hui nous allons étudier le printemps astronomique, l'équinoxe du printemps et comment il servait autrefois de base pour un système de coordonnées célestes (ascension droite/déclinaison).
Nous allons aussi vous donner un aperçu de la vie de l'astronome grec de l'Antiquité, Hipparque avant de considérer le satellite astrométrique Hipparcos et le Système de référence céleste international (SRCI) qui est aujourd'hui utilisé dans l'astronomie professionnelle au lieu du système traditionnel RA/dec.
There are two equinoxes each year, the spring one, which is termed the vernal equinox and the autumnal equinox which is in September. At these times the Sun is directly overhead (at the zenith point) as could be seen from a point on the Earth’s equator. A less technical and easier way to understand the equinoxes, and solstices (longest and shortest days) is by looking at the diagram below. (For clarity, this diagram does not show the small eccentricity of the Earth’s orbit.
The Earth revolves around the Sun once each year. The path of the Earth’s orbit around the Sun is called the ecliptic. This is also the ‘virtual’ path of the Sun when viewed from Earth. The Earth orbits the Sun in an anti-clockwise direction when viewed from above, which is the same direction as the Sun’s rotation, and is called a direct orbit.
The Earth rotates once a day and this rotation is also in the anti-clockwise direction, and so also called direct rotation. If the orbit or rotation was in the opposite direction it would be described as retrograde. However, the Earth’s axis of rotation is not at right angles to the ecliptic and the Earth has an axial tilt (which is also called obliquity) of 23°26'.
In passing, we will note that the axial tilt varies from about 22.1°to 24.5° over a cyclical period of around 41,000 years. This is due to planetary perturbations (small changes caused by gravitational forces) of the Earth’s orbit. By the year 9000AD we will have an obliquity close to 22.5°. There are also small, shorter period variations called nutation, caused predominately by the gravitational effect of the Moon.
The axial tilt of the Earth is the reason we have seasons. The axis of rotation remains pretty much fixed as the Earth orbits the Sun, and so the amount of sunlight and the length of time for which the Sun appears above the horizon (i.e. the length of the day), varies depending upon the orientation of the location on the Earth to the Sun.
So, on the far left-hand side of the picture above, the northern hemisphere is facing / leaning ‘towards’ the sun whilst the southern hemisphere is facing the sun at a lower angle. It is summer for the northern hemisphere and winter for the southern hemisphere. The Sun is both ‘higher’ in the sky, and above the horizon for a longer time, for an observer in the northern hemisphere than for an observer at the equivalent latitude in the southern hemisphere.
Another way to think of this is to spin a ball and then to tilt it, leaning the top towards you whilst keep the axis of rotation the same. The lower part of the ball (the part now furthest away from you) is visible for a shorter time than the top part; you are seeing a smaller arc on the surface of the ball at the lower part than the top part. At an extreme, when the axis of rotation of the ball is directly facing you, none of the bottom part of the ball is visible at all.
As the Earth takes its path around the Sun, with the axial tilt remaining steady, the lengths of the day and night – for all points on the Earth – converge to be of the same duration. Hence we arrive at the equinoxes (the ‘middle’ positions in the diagram above).
Historically, apart from the very real nature of the equinoxes for calendar and season identification, the equinoxes - particularly the vernal equinox - was used to define a fixed position against which to base the celestial co-ordinate system.
constellation of Pisces.
The first point of Aries, however, does provide a relatively stable longitudinal reference point for celestial coordinates. Astrometry positions are defined by a declination, i.e. how far above or below, in degrees, an object is when measured from the plane of the celestial equator*; and a right ascension, i.e. how far ‘left’ or ‘right’ of the 1st point of Aries the object is. The right-ascension (RA) axis is measured not in degrees, but in time.
(* the celestial equator is the projection of the Earth’s equator onto the celestial sphere)
So for example, the star Betelgeuse, the red supergiant star in the constellation of Orion, has celestial co-ordinates of [05h55m10s; +07°24'25'']. However, as we have noted above, the first point of Aries is also a moving point, the co-ordinates need to be referenced against a specific ‘epoch’ (which in this context is a specific time and date) and you will often find star maps referenced by the epoch for which they have been calibrated. You may also see the astrometry frame of reference defined at a specific ‘equinox’. This is a different usage of the term equinox than we have described above for vernal and autumnal equinoxes and can be thought of as a specific epoch.
Hipparchus of Nicaea
Hipparchus was one of the most prominent of Greek astronomers and mathematicians but little of his life is known. He was born in Nicaea (now called Iznik, a town in Turkey) around 190BC and died around 120BC, most likely on the Island of Rhodes. Only one of his written works has survived to the present day and much of what is known of Hipparchus is found from citations attributed to him in the later works of Greek scientists, such as Ptolemy. It is evident he was held in wide esteem both within his lifetime and afterwards and for example, images of Hipparchus are recorded on Roman coins of the second and third centuries AD. He progressed, some sources say ‘established’ the mathematics of trigonometry and, for example, constructed tables of chords to be used in solving triangle problems rather than having to work from first principles.
Hipparchus has a lasting, and undoubtedly deserved, reputation as the finest of all Greek astronomical observers and the accuracy of his observations was probably only exceeded nearly 1700 years later by the work of the Tycho Brahe. He constructed a star catalogue of around 850 stars and he determined the length of the year to within 7 minutes.
In his studies into the length of the year, he detected the precession of equinoxes, i.e. the precession of the Earth’s axial rotation, based upon both his own observations and earlier records of Greek astronomers such as Aristarchus and Meton. He most likely also made use of Babylonians’ astronomy records. The value he determined for the yearly change in direction of the first point of Aries (46 arc seconds) is astonishingly close to the value determined today (50.26 arc seconds).
The Hipparcos satellite.
On the 8th August 1989 the European Space Agency (ESA) launched an Earth orbiting satellite named Hipparcos (High Precision Parallax collecting satellite). The satellite’s mission was to provide unprecedented accuracy of the position, magnitude, proper motion and parallax (and hence distance) for over 100,000 of the brighter stars.
Today’s professional astrometry, including satellite data derived star catalogues, however does not use the traditional R.A/Dec frame of reference. The ICRS (International Celestial Reference System) uses a frame of reference based on the positions of very remote, extragalactic sources (predominately quasars) using a reference point origin based on the barycentric centre of our solar system.
Within Hipparcos’s three and a half year operations (it ceased operations in March 1993 and was deactivated in August of that year) it provided very high accuracy (within 7 milliarc seconds) for the parallaxes of just over 118,000 stars (the Hipparcos catalogue issued in 1997). The satellite also recorded less precise data (but still to a hitherto unattained accuracy of with 10 milliarc seconds) of over one million stars (published as the ‘Tycho’ catalogue, also in 1997) and the Tycho-II catalogue, again using Hipparcos data, was published data on over 2.5 million stars.
The Hipparcos mission’s objectives and results are continuing with the recent (19/12/2013) launch of the ESA Gaia satellite which has the objective of providing a 3-dimensional map of around a billion stars within our galaxy. This latter satellite is also being used to detect any Jupiter-sized planet orbiting any star with 150 light years of our own Sun. Gaia uses minute variations in the proper motions (the radial and line of site) of stars to detect the gravitational perturbations caused in the star’s motion by any orbiting planets. Results of newly detected planets are often published and occasionally you will see announcements within the mainstream media. (However, as in all things within the popular press, hyperbole and misinformation abound and it is essential to fact-check).
However, discussion of exoplanets will be reserved for a later blog post!
Our next article will be issued on the 29th of April.
More on the Life of Hipparchus
Details on the European Space Agency’s Hipparcos and Gaia satellites.