Astronomy Glossary
 
This information links to our activities
Asteroid 3200 Phaethon 
Astronomical Unit 
First Quarter 
Lunar Eclipse 
Magnitude 
Opposition 
Perihelion
Asteroid 3200 Phaethon
Discovered in October 1983 this unusual asteroid may be an 
extinct comet. It measures 5.1 Km in diameter and its orbit 
crosses the orbits of Mars, Earth, Venus and Mercury. It was the 
first asteroid to be discovered by a spacecraft.
Phaethon's most remarkable distinction is that it approaches the 
Sun closer than any other numbered asteroid.  The surface 
temperature at its closest (perihelion) could reach approximately 
1025 Kelvin. This is why it was named after the Greek myth of 
Phaëton, son of the sun god Helios.
Phaethon will approach relatively close to the Earth on December 
14, 2093, passing within 0.0198 AU (Astronomical Units).
Astronomical Unit
The astronomical unit (AU) is a unit of length approximately equal 
to the distance from the Earth to the Sun. The currently accepted 
value of the AU is 149 597 870 691 ± 30 metres (about 150 
million kilometres or 93 million miles). 
First Quarter is often referred to as half moon.  This is 
because only half of the face of the moon that we see is lit by the 
Sun.  However, half of the moon always faces towards the Sun 
and is therefore always lit by the Sun while the other half is in 
darkness (see diagram below).
 
 
 
Diagram 1: Courtesy of NASA 
(http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3
.html)
The moon orbits the Earth and therefore changes position in 
space relative to the Sun and Earth. At first quarter because of 
the relative positions of the Moon, Earth and Sun we can only 
see a portion of the lit face (see diagram of the phases of the 
moon below).
 
 
 
Diagram 2: Courtesy of NASA 
(http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3
.html)
The reason why we call this phase of the moon first quarter is 
because it is ONE QUARTER of the way through the lunar 
month. The lunar month is the number of days it takes to go from 
one NEW Moon to the next and is 29.53 days. 
Lunar Eclipse An eclipse of the moon can only occur at 
FULL MOON and only if the Moon passes through some portion 
of the Earth's shadow (see the diagram of the phases of the 
moon under First Quarter, diagram 2). 
A total eclipse happens when the entire moon passes through 
Earth's umbral shadow (see diagram below).
 
The total eclipse on March 3rd 2007 is unique in that it is partly 
visible from every continent around the world.  During the eclipse, 
the Moon is in southern Leo, about 13° east of the 1.3-magnitude 
star Regulus (alpha Leo) - see diagram below of the night sky in 
March.
 
The Moon's path through the Earth's shadow takes it through the 
northern half of Earth's umbral shadow (see diagram below).
 
Image from NASA www.nasa.gov
Although the eclipse is not central, the total phase still lasts 73 
minutes.  The timings of the major phases of the eclipse are 
listed below:
Penumbral Eclipse 
Begins
20:18:11
P1
Partial Eclipse Begins
21:30:22
U1
Total Eclipse Begins
22:44:13
U2
Greatest Eclipse
23:20:56
Greatest
Total Eclipse Ends
23:57:37
U3
Partial Eclipse Ends
01:11:28
U4
Penumbral Eclipse 
Ends
02:23:44
P4
UT = Universal Time, which at the moment is the same time as 
UK time.
Since the Moon samples a large range of umbral depths during 
totality, its appearance will change dramatically with time.  
However, it is not possible to predict the exact brightness 
distribution in the umbra.
During totality, the spring constellations will be well placed for 
viewing so a number of bright stars can be used for magnitude 
comparisons.  Spica (mv = +0.98) is 40° southeast of the 
eclipsed Moon, while Arcturus (mv = -0.05) is 49° to the 
northeast.  Procyon (mv = -0.05) is 50° to the west.  Saturn 
shines at magnitude +0.8 about 24° northwest of the Moon near 
the western border of Leo .
Magnitude is a measure of how bright a celestial object 
looks. Those objects that can be seen with the naked eye are 
ranked in 6 magnitudes from first to sixth magnitude. First 
magnitude is the brightest and 6th magnitude the faintest, which 
always seems a little odd! Anyway a sixth magnitude object is 
exactly 100 times less bright than a first magnitude object. This 
means that the difference between a first and second magnitude 
object is approximately 2.51 times. To get the difference between 
a first and second magnitude object all you do is multiply 2.51 x 
2.51 = 6.3.This means that a third magnitude object is about 6.3 
times less bright than a first magnitude object. 
To make things a little more complicated, an object 2.51 times 
brighter than magnitude 1 becomes magnitude 0. An object 6.3 
times brighter than magnitude 1 becomes magnitude -1.
Sirius is the brightest STAR in the sky and has a magnitude of -
1.44. The full Moon has a magnitude of -12.7 and the Sun has a 
magnitude of -26.7.
Opposition is the time when a celestial body, in this case 
Saturn, is on the opposite side of the sky to the Sun (see diagram 
below). 
 
Diagram courtesy of NASA
The orbit of Saturn brings it closer to Earth making it appear 
larger and brighter. On the 10th February Saturn will rise when 
the Sun sets and will set when the Sun rises so will be visible all 
night.
 
Perihelion
All of the planets in our Solar System move around the Sun in 
elliptical orbits. An ellipse is a shape that can be thought of as a 
"stretched out" circle or an oval as in the diagram below. The Sun 
is not at the centre of the ellipse, as it would be if the orbit were 
circular. Instead, the Sun is at one of two points called "foci" 
(which is the plural form of "focus") that are offset from the 
centre. This means that each planet moves closer towards and 
further away from the Sun during the course of each orbit. The 
point in the orbit where the planet is closest to the Sun is called 
"perihelion".  The point in the orbit where the planet is furthest 
away from the Sun is called "aphelion".