Understanding Planetary Observation Terminology
Reading about the positions of
the planets each month, there are some words which crop up which can throw
people into confusion. At times it can be like reading another language! So here
is a brief summary of the commonly used terms and what they mean. The diagram
will help you to better understand these descriptions!
Orbits
In our solar system, the
planets orbit the Sun and each planet has moons which are in orbit around them.
For simplicity, the orbits are often depicted as being circular, but in fact
most of them are actually oval shaped, or elliptical. With the planets, the
further away from the Sun you go, the more elliptical the orbit is. This means
that there are times when the orbiting body is closer to its parent and times
when it is further away. There are 2 terms used to describe this. Perihelion
is the point where the orbiting body is closest to its parent, and aphelion
is its furthest point. Our Moon also has an elliptical orbit, but the
Moon has its own special words to describe its nearest and furthest points in
its orbit. Perigee is when the Moon is at perihelion, and Apogee
is when it is at aphelion. Planets which are orbiting closer to the Sun
than Earth, i.e., Mercury and Venus, are called inferior planets;
those which orbit further away from the Sun than Earth are called superior
planets.
Prograde and Retrograde
Motion
If you study the planets from
Earth over a number of nights, they appear to move roughly along an invisible
line called the ecliptic. Most of the time they appear to move
from east to west over a number of nights, and this “forward” movement is
called prograde motion. However, there are times when a planet
appears to slow down, stop, and then move backwards over a number of nights.
This apparent “backwards” movement is called retrograde motion.
The planets aren’t really moving backwards, they just appear to be from here on
Earth. Because Earth is closer to
the Sun its orbit is smaller, so sometimes it will align with the superior
planet (the point where it appears to stay still) then overtake it on the
inside (the point where the superior planet is in retrograde motion). After a
while the planets realign themselves and the movement once again become
prograde.
Opposition and Conjunction
(Superior Planets)
Opposition refers to the point in a superior planet’s orbit where
it is on the opposite side of the Earth to the Sun, forming a straight line
between the three bodies. When a planet is at opposition it is receiving the
maximum amount illumination from the Sun, therefore is usually the best time
for observation.
Conjunction is when 2 objects appear close to each other when
viewed from Earth. This holds true when referring to planetary orbits. When
viewed from Earth, a planet at conjunction is in the same part of the sky as
the Sun. In order for this to occur, the planet needs to be on the opposite
side of the Sun to the Earth. When a superior planet is at conjunction it is
usually unfavourable for observation because it will appear close to the sun.
Superior Conjunction and
Inferior Conjunction (Inferior Planets)
Because their orbits are inside
that of the Earth, it is not possible for inferior planets to reach opposition.
Instead, there are 2 points in their orbit where they are in conjunction.
Superior conjunction is when the planet makes a straight line with the Sun
and Earth, but is on the opposite side of the Sun to the Earth.
Inferior conjunction is when the planet makes a straight line with the Sun
and Earth, but is located in between the Sun and the Earth.
It is difficult to observe the inferior planets when they
are in either conjunction because they appear too close to the Sun.
Greatest Elongation
(Inferior Planets)
This is the point in the orbit
of inferior planets when they are best placed for observation.
Greatest Eastern
Elongation is the point where the
planet is at its furthest point east from the Sun when viewed from Earth. When
at greatest eastern elongation a planet will be an evening object, visible
after sunset.
Greatest Western Elongation is the point where the planet is at its furthest
point west from the Sun when viewed from Earth. When at greatest western
elongation a planet will be an early morning object, visible before sunrise.
A note about magnitudes
The magnitudes of stars can
appear to be confusing at first. It is a scale which was first devised by
ancient astronomers about 2000 years ago and it refers to how bright a star
appears to be as viewed from Earth. Contrary to what you might expect, the
brighter the star, the lower the magnitude number will be and the dimmer the
star, the higher its magnitude number will be. This is because those early
astronomers categorised stars by their relative apparent brightness; “1st class” stars
being 1st magnitude, going down to the faintest stars which were “6th class”, or 6th magnitude.
Originally they stopped at 6 because that is the limit of what the human eye
can see as viewed from a dark sky site. In the present day, stars are still
classified using this old terminology. It is a logarithmic scale and a
difference of 1 order of magnitude is 2.5 times difference in apparent
brightness. So a magnitude 1 star is 2.5 times brighter than a magnitude 2
star, for example. The bright summer star, Vega, was used as a starting point
when this scale was devised, having a magnitude of 0. Anything brighter than
Vega has a negative magnitude number, for example, Sirius the Dog Star has a
magnitude of -1.5.
When looking at planetary data,
it is common to see Venus and Jupiter having negative magnitudes because they
are usually brighter than the brightest stars. Iridium flares and the
International Space Station are regularly mag -3 or brighter. Just for
comparison, Polaris the Pole Star, has a magnitude of 2, whereas the Full Moon
has a magnitude of -13, and the Sun has a whopping value of -27!
It can take a while for these definitions to sink in, but
hopefully this will make the monthly planetary data easier to understand.
This comment has been removed by a blog administrator.
ReplyDeleteHi Mary ! I'm just curious about this line "It only happens with the superior planets." in reference to retrograde motion. Is there a separate term for the motion of Mercury & Venus ?
ReplyDeleteHi, thank you so much for point this out because it's actually an incorrect statement. I've now removed that line from the blog. I think in my brain I was referring to opposition, which is something that can't happen with Mercury and Venus.
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