Astronomy Textbook

[Kaiia Blackthorn]

This is text we will be working from this year.

When a class is announced, please note the chapters that you are required to read before the class.

The curriculum will include:

Constellations
The moon
The solar system
Small solar system bodies
Stellar evolution



GLOSSARY OF TERMS
(Will be updated as new chapters are added.).

asterism - a pattern of stars that is not an official constellation.
Often, an asterism is part of a large constellation.

ecliptic - the plane of the earth's orbit around the sun.

galaxy - a massive system of stars, gas, dust and other interstellar materials, that are gravitationally bound and orbit a common centre of mass.

lunation - one complete set of lunar phases, from one New Moon to the next New Moon.
This takes approximately 29 days.

node - ascending & descending:
the points at which the orbit of the moon around the earth intersects with the orbit of the earth around the sun.
Ascending means that the moon passes from below to above the ecliptic
Descending is the opposite - the moon passes from above to below the ecliptic.

penumbra - the outer, lighter area of shadow cast by one object on another

umbra - the central darker shadow cast by one object on another.




OOC:
Sources
home.hiwaay.net/~krcool/Astro/moon/
en.wikipedia.org/

[Kaiia Blackthorn]

What Are Constellations?
A constellation is a set of stars identified as a distinct grouping.

These groupings are base purely on how they appear to
observers on earth. However the stars within a constellation
may be nowhere near each other when their relative distances
are considered.

Asterisms
Some constellations contain smaller groupings of stars
that are well known. Such groupings are called asterisms.

For example The Big Dipper is not a constellation;
it is an asterism within the constellation Ursa Major.

The system of constellations used today has its roots in
ancient Greek and Roman civilizations. Many of the
constellations are named after mythological gods, heros
and animals. The ancients imagined they could see these
creatures depicted in the patterns formed by the stars.

There are a total of 88 modern constellations, but they
cannot all be seen from any one location on earth.
This text will use six divisions; the northern and southern
polar regions, and four equatorial regions.

Star Identification
Bright stars within constellations are identified by Greek
letters; many are also named. Those without names are
identified by the Greek letter plus the genitive form of the
constellation name.

For example the delta star in Ursa Major is referred to
as Delta Ursae Majoris

Greek Alphabet

Symbol

α

β

γ

δ

ε

ζ

η

θ

ι

κ

λ

μ

Name

alpha

beta

gamma

delta

epsilon

zeta

eta

theta

iota

kappa

lambda

mu

Symbol

ν

ξ

ο

π

ρ

σ

τ

υ

φ

χ

ψ

ω

Name

nu

xi

omicron

pi

rho

sigma

tau

upsilon

phi

chi

psi

omega

Constellations of the Northern Polar Region
Constellations that lie above 60°N are listed as N.P. (North Polar)
For Constellations that lie below 60°N, the optimal viewing season is stated.

Scientific Name	Genitive	Common Name	Season	Named Stars
Auriga	Aurigae	Charioteer	Winter	Capella, Menkalinan
Boötes	Boötis	Herdsman	Spring	Arcturus, Nekkar, Haris, Izar, Muphrid
Camelopardalis	Camelopardalis	Giraffe	N.P.	none
Cassiopeia	Cassiopeiae	Queen	N.P.	Schedar, Caph, Rucbar, Cih, Segin
Cepheus	Cephei	King	N.P.	Alderamin, Alfirk, Alrai
Cygnus	Cygni	Swan	Summer	Deneb, Albireo, Sadir, Gienah
Draco	Draconis	Dragon	N.P.	Thuban, Alwaid, Etamin, Nodus II, Nodus I, Kuma
Lacerta	Lacertae	Lizard	Autumn	none
Lynx	Lyncis	Lynx	Winter	none
Perseus	Persei	Hero	Winter	Mirfak, Algol, Menkhib
Ursa Major	Ursae Majoris	Great Bear	Spring	Dubha, Merak, Phad (Phekda), Megrez, Alioth, Alkiad (Benetnasch), Mizar
Ursa Minor	Ursae Minoris	Little Bear	N.P.	Polaris, Kochab, Pherkab

[Kaiia Blackthorn]

Genitive form: Ursae Majoris
Abbreviation: UMa

Ursa Major is the third largest constellation. It contains one of
the best known asterisms - the Plough aka Big Dipper



The stars that make up the Plough are:
α - Dubhe
β - Merak
γ - Phad (Phekda)
δ - Megrez
ε - Alioth
ζ - Mizar
η - Alkaid (Benetnasch)

The brightest stars in Ursa Major are Dubhe and Alioth

Ursa Major is seen at its highest point in the sky from
February - May. It is fully visible between 90°N and 16°S

Interesting deep space objects that can be observed within Ursa Major:

The M81 Group
This is a group of galaxies that can be viewed in Ursa Major. It
contains a number of galaxies, the most notable being M81, M82
and NGC 3077

M81 - Bode's Galaxy



M81 is a spiral galaxy that is seen at an angle, and is
approximately 12 million light years away. It was discovered
on Dec 31 1774 by Johann Elert Bode

M82 - Cigar Galaxy



M82 is a spiral galaxy seen edge-on. It is also classified as a
starburst galaxy - one that has an unusually high rate of star
formation. In the case of M82, this is thought to be caused by
gravitational forces from nearby M81. M82 is approximately
12 million light years from earth.

NGC 3077



This elliptical galaxy was discovered by William Herschel on
November 8 1801. It is affected by gravitational interaction
with M81 and M82.

M97 - the Owl Nebula



A planetary nebula, discovered on Feb 16 1781. It is about
2600 light years from earth. It is called the Owl Nebula
because it has two dark areas that give it the appearance of
eyes.

M101 - Pinwheel Galaxy



A beautiful spiral galaxy, seen face-on, that is about 27
million light years away. It was discovered on March 27 1781
by Pierre Mechain. It is almost twice as big as our own galaxy.

M108



An edge-on spiral galaxy, 40 - 50 million light years away. The spiral structure is not clearly delineated, and there is no strong centre, and very little bulge. It is described as being a very dusty galaxy.

M109



This is the main galaxy in the M109 group, which may contain over 50 galaxies. It is a barred spiral galaxy, located approximately 46 million light years away. In March 1956, a supernova was observed in the southeast part of the galaxy.


Using Ursa Major as a Pointer
Once you have located Ursa Major in the sky, it can be used as
a pointer to a number of other constellations.



Pointer to Ursa Minor and Cepheus
Draw a line between Merak and Dubhe; extend that line
upwards, to locate Polaris in Ursa Minor, and beyond that,
Alrai in the constellation Cepheus.

Pointer to Cygnus and Leo
Draw a line between Phad and Megrez; extend the line up to
Albireo in the constellation Cygnus; extend it down to Regulus,
in the constellation Leo.

Pointer to Cassiopeia
Draw a line between Mizar and Polaris (in Ursa Minor); extend
the line to Ksora in the constellation Cassiopeia.

Pointer to Capella
Draw a line between Phad and Dubhe; extend the line to
Capella in the constellation Auriga

Pointer to Gemini
Draw a line between Megrez and Merak; extend the line to
Pollux in the constellation Gemini.

Pointer to Bootes and Virgo
Draw a line between Mizar and Benetnasch; extend a line
curving down to Arcturus in Bootes; continue that line down
to Spica in Virgo.

[Kaiia Blackthorn]

STATISTICS
Size: 3474 km diameter
Distance From Earth: 384,403 (average)
Lunation: 29.5 days


ORIGINS OF NAMES FOR THE MOON
The word 'moon' is Germanic. It is related to the Latin word
mensis, and name derives from the proto-Indo-European
root me- which is also represented in words like measure,
mean ('average') and menstrual.

The Latin name for the moon is Luna - the adjective of the
name is lunar.

From the Greek deity Selene comes the prefix seleno- as in
selenography (the study of the moon's surface and physical
features) and selenolgy (science and geology of the moon).

THE TWO FACES OF THE MOON
The moon is in synchronous rotation - the speed of the rotation
combined with the speed of the moon's orbit around earth
means that the moon keeps the same face turned towards
earth at all times. However, it is actually possible to see about
59% of the moon's surface from earth (though only 50% at any
one time) due to slight variations in the angle from which we
view the moon.

The surface temperature of the moon ranges from 107^o celcius
on the day side to -153^o celcius on the night side.

Although only one face of the moon is visible from earth, muggle spaceships have orbited the moon and taken pictures of the far side, as seen below.



MARIA, TERRAE AND CRATERS
There are three main features visible from earth. Maria
(singluar mare) are the dark lunar plains - these are actually
vast pools of ancient basaltic lava, now solidified. Terrae are
the higher mountainous areas of land. These mountains were
not formed by tectonic activity but are thought to be the
ancient remnants of the outer rims of impact craters.

Impact Craters are the scars left by the collision of asteroids
and comets. These craters are relatively well-preserved,
because the moon lacks the atmosphere, weather and tectonic
events that have eroded or destroyed most of the similar
impact craters on earth.



Mare
Mare Frigoris - sea of cold
Mare Serenitatis - sea of serenity
Mare Tranquilitatis - sea of tranquility
Mare Crisium - sea of storms
Mare Foecunditatis - sea of fertility
Mare Nectaris - sea of nectar
Mare Vaporis - sea of vapours
Mare Imbrium - sea of rains
Oceanus Procellarum - ocean of storms
Mare Humorum - sea of moisture
Mare Nubium - sea of clouds

Impact Craters
Plato
Copernicus
Tycho


PHASES OF THE MOON
The moon does not 'shine'. What we perceive as moonlight is
actually reflected sunlight. At any one point in time, half of
the moon is lit up, and half is in darkness. During the moon's
revolution around the Earth, we see a varying proportion of
the sunlit side, depending upon the earth's orientation in
relation to the moon.

Lunation

From one full moon to the next takes approximately 29 days - this period is called a lunation.

New Moon
When the moon is between the Earth and the Sun, the portion
of the moon that is illuminated is facing away from the earth,
so we cannot see any part of the moon. This is called the New
Moon.


Waxing Crescent
For approximately 14 days following a new moon, the moon
waxes - this means that it grows. A few days after the new
moon, the first sliver of moon is seen. This is the Waxing
Crescent. The points of the crescent point to the east.


First Quarter
When the Earth is perpendicular to the moon, we can see half
of the illuminated portion. This is sometimes called a Half
Moon. Confusingly, however, the official term is First
Quarter. It is called this, because the moon is now a quarter of
the way through a lunation.


Waxing Gibbous
A gibbous moon is seen between First Quarter and Full Moon,
and then between Full Moon and Last Quarter.


Full Moon
At Full Moon, the Earth is between the sun and the moon, so
we can see the entire illuminated side of the moon.


Waning Gibbous
Between Full Moon and the next New Moon, the illuminated
portion visible to the Earth wanes, or grows less.


Last Quarter
Also known as the Third Quarter. As with First Quarter, the
name refers to the stage in a lunation, not to the shape of the
moon as we can see it.


Waning Crescent
A waning crescent is similar to a waxing crescent, but the
points of the crescent now face west. An easy way to
remember whether you are looking at a waxing or waning
moon is that if the moon appears to form a letter 'C' then it is
Contracting, or waning.

[Kaiia Blackthorn]

TELLING THE TIME BY THE MOON
Because moonrise takes place about 50 minutes later each day
than the day before, it is possible to tell the time from the
position of the moon

* A New Moon always rises at sunrise.
* A First Quarter Moon always rises at noon.
* The Full Moon always rises at sunset.
* The Last Quarter Moon always rises at midnight.


CALCULATING THE DATE OF EASTER

A rough approximation of the method for determining the
date of Easter is that it is 'the first Sunday after the full moon
that occurs next after the vernal equinox'. This is a little
misleading, as there is a precise set of rules which determines
when Easter occurs each year.

The actual conditions to determine the date for Easter are:
(1) Easter must be on a Sunday.
(2) This Sunday must follow the 14th day of the paschal moon.
(3) The paschal moon is that of which the 14th day (full
moon) falls on or next follows the day of the vernal equinox.
(4) The equinox is fixed in the calendar as March 21.

These rules mean that Easter can never occur earlier than
March 22 or later than April 25.


FULL MOON NAMES
Many cultures assign a name to each Full Moon - different
cultures will have their own set of names, depending upon
their climate and environment.

January - Wolf Moon
February - Ice or Snow Moon
March - Storm or Worm Moon
April - Growing or Pink Moon
May - Hare or Flower Moon
June - Mead or Strawberry Moon
July - Hay or Buck Moon
August - Corn or Sturgeon Moon
September - Harvest Moon *
October - Blood or Hunter's Moon
November - Snow or Beaver Moon
December - Cold Moon

*The Full Moon closest to the Autumn Equinox (Mabon)
is generally referred to as the Harvest Moon - so the Harvest
Moon will sometimes occur in October.


BLUE MOONS AND BLACK MOONS
The common definition of a blue moon is the occurrence of two
full moons in the same calendar month. The Moon is full
every 29 ¹/₂ days, so it’s possible to
have two full moons in any month except February. On
average, there is only one Blue Moon every 33 months.

However, this definition is believed to be a misunderstanding
of the original meaning, which was four full moons in a single
season. Almanacs showing the phases of the moon often divide
the year into four seasons, each season usually having three
full moons. Each full moon is referred to as Early, Mid or Late
season (for example 'Mid Winter Moon). When four full moons
occur in a season, in order to keep the labelling correct, the
third full moon in that season is called a Blue Moon.

Blue Moons occur
*7 times every 19 years.
* 37 times every century.
* Once every 33 full moons.

Black Moons
What is more rare than a blue moon? A month with no full
moon. This phenomenon is referred to as a 'Black Moon', and
happened only four times this past century, the last in
February 1999. February is the only month in which
this can occur. The month before and the month afterwards
will both have blue moons. The 21st century will see the event
only four times: February 2018, 2037, 2067, and 2094.

[Kaiia Blackthorn]

ECLIPSES
An eclipse occurs when the moon's shadow falls on the Earth,
or the Earth's shadow falls on the moon. In each case, the
shadow has two parts. The umbra is a central cone of
darkness. It tapes away from the body casting the shadow.
The penumbra is the outer cone of partial shadow. It
diverges from the body casting the shadow. The type of eclipse
seen – whether lunar or solar – depends on what part of the
shadow reaches the earth or moon.

Solar Eclipses
NOTE: You should NEVER look directly at the sun, even during an eclipse.
People have lost their sight just by looking at the sun.

A solar eclipse occurs when the moon is between the earth and
the sun, and casts a shadow on the earth.



There are three types of solar eclipse.
Total Eclipse: An observer on the area on the Earth that is covered
by the umbra will see a total eclipse.
Partial Eclipse: People in the area covered by the penumbra will see
a partial eclipse. Anyone outside the penumbra will not see
the eclipse.
Annular Eclipse: Due to variations in its orbit, the moon may be
slightly further from the earth than average, so the shadow
does not actually touch the earth. However, the moon still
blocks part of the light, so we see a dark circle surrounded by
the visible portion of the sun.

Path of Totality
As the Earth rotates, the shadow will travel across the earth
along a track that is usually curved. This path is known as
the Path of Totality. This path may be thousands of kilometers
long, but can never be more than 270 km wide.


Lunar Eclipses
A lunar eclipse is seen by anyone who can see the moon at the
time of the eclipse.


Because the shadow cast by the earth is much larger than the
shadow cast by the moon on the earth, the umbra can cover
the entire disc of the moon at the same time. It is also possible
for the moon to pass through the penumbra but not the
umbra. There are therefore four types of lunar eclipse:

Total Umbral: The entire moon passes through the umbra
Partial Umbral: Only part of the moon passes through the umbra,
the remainder only passes through the penumbra.
Total Penumbral: the entire moon passes through the penumbra,
but does not pass through the umbra
Partial Penumbral: only part of the moon passes through the penumbra

Appearance of the moon during a lunar eclipse
The umbra cast by the earth is not completely dark, because
the earth's atmosphere refracts some light into the umbra and
therefore onto the surface of the moon. As a result, the
appearance of the moon during an umbral eclipse may vary
from very dark and difficult to see, to a bright orange-red,
depending on atmospheric conditions

Similarly, penumbral eclipses may very from pale amber to
extremely pale yellow which may be almost undetectable.

Moon During Eclipse
This composite photograph shows the progression of a total
lunar eclipse.


The Danjon Scale
This scale, developed in the 1920s by Andre Danjon,
categorizes the depth of an umbral lunar eclipse by appearance.
The scale is as follows:

L=4: Bright copper-red or orange umbra, with bluish rim.
L=3: Brick-red umbra, with light grey or yellow rim.
L=2: Deep red or rust-coloured umbra, with dark centre and brighter rim.
L=1: Dark grey or brownish umbra. Difficult to see features.
L=0: Very dark umbra. Moon itself difficult to see.

Why don't eclipses occur at EVERY New and Full Moon?
A solar eclipse can only happen at New Moon, and a lunar
eclipse can only happen at Full Moon.
If the moon orbited the earth on exactly the same plane as the
Earth orbits the sun, then there would be a solar eclipse at
every New Moon and a Lunar Eclipse at every Full Moon.
However the plane of the moon's orbit is inclined about 5
degress to the plane of the ecliptic (the Earth's orbit).



Therefore, eclipses can only occur when the following
conditions are met: 1) The sun, earth and moon are linearly
aligned AND 2) The moon is crossing the ecliptic. If the moon
is not crossing the ecliptic, it will be slightly above or slightly
below the plane of the ecliptic, and it will not pass between the
earth and the moon.

[Kaiia Blackthorn]

THE MOON'S EFFECT ON TIDES[/u]

"Tides" refers to the rise and fall in sea level with respect to the
land, produced by the gravitational attraction of the moon
and the sun. The same forces also cause tides to occur in large
lakes, the atmosphere, and even within the crust of the earth,
but to a much smaller extent.

What are Lunar Tides
The Earth and the moon are attracted to each other. The
moon’s gravity pulls at anything on the Earth. Earth is able to
hold onto everything except the water - water is always
moving, so the Earth cannot hold onto it, and the moon is able
to pull it closer. Every day, there are two high tides and two
low tides - the ocean is constantly moving from high to low
tide, and then back again. There is about 12 hours and 25
minutes between the two high tides.

The gravitational attraction of the moon causes the oceans to
bulge out in the direction of the moon. Another bulge occurs
on the opposite side, since the Earth is also being pulled toward
the moon (and away from the water on the far side). Ocean
levels fluctuate daily as the sun, moon and earth interact. As
the moon travels around the earth and as they, together,
travel around the sun, the combined gravitational forces
cause the world's oceans to rise and fall. Since the earth is
rotating while this is happening, two tides occur each day.

What are the different types of Tides?

Spring Tides
When the moon is full or new, the gravitational pull of the
moon and sun are combined. At these times, the high tides are
very high and the low tides are very low. This is known as a
spring high tide. Spring tides are especially strong tides (they
do not have anything to do with the season Spring) which
occur when the Earth, the Sun, and the Moon are in a line.
The gravitational forces of the Moon and the Sun both
contribute to the tides. Spring tides occur during the full moon
and the new moon.

Neap Tides
During the moon's quarter phases the sun and moon work at
right angles, causing the bulges to cancel each other. The
result is a smaller difference between high and low tides and is
known as a neap tide. Neap tides are especially weak tides.
Neap tides occur during quarter moons.

The Proxigean Spring Tide
This is a rare, unusually high tide. This very high tide occurs
when the moon is both unusually close to the Earth (at its
closest perigee, called the proxigee) and in the New Moon
phase (when the Moon is between the Sun and the Earth). The
proxigean spring tide occurs at most once every 1.5 years

THE SEQUENCE OF TIDES DURING A LUNATION

[Kaiia Blackthorn]

DEFINITION
The solar system is the region of space within the gravitational influence of the sun.

The Solar System is about 15 trillion km (9.3 trillion miles) in diameter.

It includes the following:
- the Sun
- 9 planets **
- over 140 moons
- innumerable small bodies including asteroids and comets



Inner Region – The Rocky Planets
- Mercury
- Venus
- Earth
- Mars

Asteroid belt
- lies between Mars and Jupiter

Gas giants
Jupiter
Saturn
Uranus
Neptune

Outer region
- Pluto
- ice dwarfs
- Kuiper Belt
- Oort Cloud


THE FORMATION OF THE SOLAR SYSTEM



1 – Formation of the Solar Nebula
The solar system originated as a vast cloud of cold gas & dust
that was much larger than the present solar system. It was
spinning very slowly. The initial temperature of the cloud
was about -230C (-382F)

2 – Formation of the Protosun
The solar nebula condensed due to gravity into the protosun
(the dense central region) and the protoplanetary disk (the
diffuse outer region).
As it contracted, 3 things happened:
a) the disk began spin faster
b) the cloud flattened out
c) the central region started heating up

3 – Formation of Rings and Planetesimals
Unstable regions within the protoplanetary disk condensed
into rings, and within these rings, accretion of smaller
particles formed small rocky or icy objects called planetesimals

4-5 – Formation of the Planets
Planetesimals collided and combined to form planets. Close to
the protosun, only rocky materials and metals could survive
the heat, therefore the planets closest to the sun are composed
of these materials. In the outer region of the disk,
planetesimals were made of rock and ice and attracted large
amounts of gas around them, forming the gas giant planets.
Soon after this, the protosun finally became a star.

Most of the remaining gas and unaccreted material was blown
away by solar radiation. Leftover planetesimals in the outer
part of the disk formed the Oort cloud.


BASIC STATISTICS ABOUT THE PLANETS

PLANET	DISTANCE FROM SUN	ORBITAL PERIOD	ROTATIONAL PERIOD	DIAMETER	MASS *	SURFACE TEMP	MOONS
Mercury	58 million km (36 million miles)	88 days	59 days	4875 km (3029 miles)	0.055 e.m.	-180oC to 430oC (-290oF to 810oF)	0
Venus	108 million km (67 million miles)	225 days	243 days	12,104 km (7521 miles)	0.82 e.m.	464oC (867oF)	0
Earth	149.6 million km (93 million miles)	365.26 days	23.93 hrs	12,756 km (7926 miles)	1 e.m.	15oC avg (59oF avg)	1
Mars	228 million km 141.6 million miles	687 days	24.63 hrs	6780 km 4213 miles	0.11 e.m.	-125oC to 25oC (-195oF to 77oF)	2
Jupiter	778 million km (483 million miles)	11.86 years	9.93 hrs	142,984 km (88,846 miles)	318 e.m.	-110oC (-160oF)	63
Saturn	1.4 billion km (886 million miles)	29.46 years	10.66 hrs	120,536 km (74,898 miles)	95 e.m.	-140oC (-220oF)	34
Uranus	2.9 billion km (1.8 billion miles)	84.01 years	17.24 hrs	51,118 km (31,763 miles)	14.5 e.m.	-214oC (-353oF)	27
Neptune	4.5 billion km (2.8 billion miles	164.8 years	16.11 hrs	49,532 km (30,760 miles)	17.1 e.m.	-200oC (-320oF)	13
Pluto **	5.9 billion km (3.7 billion miles)	248.6 years	6.38 days	2,304 km (1432 miles)	0.004 e.m.	-230oC (-382oF)	1 **

* Mass as compared with earth - e.m. = 'Earth Masses'
**Information correct as of 2003 (before the discovery of two
additional moons orbiting Pluto, and before Pluto itself was
'voted off the island'!)

[Kaiia Blackthorn]

The term 'small solar system bodies' covers all objects in the
solar system that are not planets or dwarf planets. It includes
asteroids, comets and meteors. It is interesting to note that
although wizard astronomers are familiar with this term, it
has not yet been adopted by Muggles. Hopefully they will
catch up soon!


Diagram of the Solar System, Showing the Kyuiper Belt
and the Hypothetical Oort Cloud

Asteroids
Asteroids are believed to be the remnants of a failed planet. They are is too small to have an atmosphere. Over 200,000
are known, but it is estimated that over a billion may exist.

The Structure of Asteroids
There are three main classes of asteroids based on their
composition. Carbonaceous, silicaceous and metallic. These
groups may also be referred to as stony, stony-iron and iron.

The surface of an asteroid is pitted with craters, from collisions
with other asteroids. In some cases, numerous collisions with
tiny asteroids have created areas that have a sand-blasted effect

Asteroids vary widely in size. Only about 100 are larger than
200 km in diameter. Around 100,000 are between 20 and
200 km in diameter. There is thought to be over 1 billion
asteroids between 2 and 20 km in diameter, and countless
asteroids smaller than 2 km in diameter.

Asteroid Orbits
There are several distinct groups of asteroids. All groups stay
close to the plane of the solar system, and orbit in the same
direction as the planets.


Inner Solar System Asteroids

Inner Solar System Asteroids

Vulcanoid asteroids
This is a hypothetical group - as yet, none have been
discovered. If they exist, they would orbit entirely within the
orbit of Mecury.

Apoheles
These asteroids orbit entirely within the Earth's orbit. They
include Mercury-crossers and Venus-crossers, whose orbits
cross the orbit of Mercury or Venus. All known
Mercury-crossers are also Venus crossers.

Earth-Crossers
These are asteroids that cross the orbit of Earth. They spend
part of their orbit outside that of Earth, and part inside Earth's
orbit. They include the Apollo and Aten Groups. These
groups, along with those in the Amor group, are known
collectively as near-Earth asteroids. There is a very small,
possibility of asteroids in these groups hitting Earth. Apollo
group asteroids have orbital paths that cross the paths of both
Mars and Earth. Aten asteroids occur mostly inside the orbit
of Earth.

Amor Asteroids
At their closest approach to the sun, these asteroids are just
outside the orbit of Earth.

Mars-Crossers
Their orbits cross that of Mars. Some, but not all, also cross the
orbit of Earth

Main Belt
The majority of asteroids are found in the Main Belt, a
concentration of asteroids between the orbits of Mars and
Jupiter. The typical orbital period of these asteroids is
between 4 and 5 years.

Collisions between asteroids occur frequently, causing them to
break up over time. The largest Main Belt asteroid is 4 Vesta,
with a diameter of 560 km. It is the brightest asteroid, and
the only one that is visible from Earth with the naked eye.

Between the Main Belt and Jupiter are a number of asteroid
groups that are distinguished by average distance from the
sun, or combinations of several orbital elements.

Trojans
Trojans are asteroids that have the same orbital path as a
planet, either following or leading the planet. Planets with
Trojans include Mars, Jupiter and Neptune. Earth Trojans
are currently hypothetical only.


Asteroids to the Orbit of Jupiter

Trans-Neptunian Objects
These have a mean orbit greater than 30 AUs. This group
includes Kuiper Belt Objects and the hypothetical Oort cloud.

Kuiper Belt Objects
This group includes objects that lie between 30 and 50 AUs.


COMETS

Composition
The central body of a comet is composed mainly of ice and
small silicate dust particles. One description of a comet is that
it resembles a “dirty snowball”. The ice is mainly water, but
also includes such substances as carbon dioxide, carbon
monoxide, methane and ammonia.


The Composition of a Comet

Comet Coma and Tail
When comets enter the inner solar system, comets develop a
coma - a cloud of dust and gas, as much as 100,000 km across.
As a large comet approaches the sun, it develops two tails. One
is straight, and is composed of ionized gas that is being blown
away from the coma by solar wind. The second is curved, and
is made of dust being pushed away by solar radiation. Because
this material is pushed outwards away from the sun, a comet
always appears to point towards the sun, with the tails
stretching out behind it.


As the comet passes the sun, the tail always stretches away
from the sun

Cometary Orbits
Comets have highly-elliptical orbits that bring them very
close to the sun at perihelion. Some comets have a very
predictable orbital period, others may have extremely erratic
orbits - passing close to the gravitational fields of the giant
planets may affect the comet's trajectory enough to
significantly change its orbital period.


A comparison five comets with different orbital periods:
Halley (76 years); Hale-Bopp (2400 - 4200 years); Hyakutake (30,000 years);
Kohoutek (75,000 - 150,000 years); West (500,000 years)

Short Period Comets
Short Period comets that do not go outside the orbit of Jupiter
have orbital periods of about 7 years, though some are much
shorter, such as Encke, which has an orbital period of 3.3 years.




Comet Encke and its Orbit

Long Period Comets
Intermediate and Long Period comets have orbital periods
longer than 20 years. The best known example is Halley's Comet, with an orbital period of 76 years.


Halley's orbit takes it outside that of Neptune

Some Long Period comets have orbital periods of hundreds of
thousands of years. Comet West, for example, was at its closest
approach to the sun in February 1976, and will not approach
the sun again for 500,000 years.


Comet West

Sungrazers
This is the name given to comets that pass extremely close to
the sun at perihelion. Some small sungrazers evaporate
completely, but larger ones can survive many passages,
though they often fragment. Kreutz sungrazers are thought to
be fragments of a single comet that broke into pieces several
centuries ago. The brilliant comet Ikeya-Seki was one of these.
However, not all sungrazers are part of the Kreutz group.


Sungrazer Comet Ikeya-Seki

Great Comets
Although there is no official definition of a great comet, the
term is applied to exceptionally bright comets. For example,
Hale-Bopp was bright because of its large nucleus. Hyakutake,
on the other hand, had a smaller nucleus, but appeared bright
because it passed within 15 million km of Earth



Great Comets Hale-Bopp and Hyakutake

Notable Comets

Name	Period (years)	Last Visit	Next Visit	Perihelion (km)	Notes
Great Comet of 1680	9400	1680	in about 9100 yrs	940,000	Sungrazer (not a Kreuz comet). The first comet to be discovered by telescope. Was so bright that it was visible during daylight. It was used by Isaac Newton to test and verify Kepler's Laws of planetary motion.
Ikeya-Seki	184	1965	c. 2149	470,000	Sungrazer. Ikeya-Seki was so bright at perihelion, on Oct 21 1965, that it was visible to the naked eye next to the sun at noon, and was brighter than the full moon.
Halley's Comet	76	1986	2062	88 million	First recorded sighting was in 240 BC. It was the first period comet to be identified as such. In 1696, Edmond Halley reported that comets seen in 1531, 1607 and 1682 were in fact the same comet. He predicted that the comet would be seen again in 1758. He was proved correct.
Shoemaker- Levy 9	-	1994	-	-	This comet was not orbiting the sun, but Jupiter. It had been fragmented into 22 pieces during its July 1992 perihelion. In July 1994, muggle telescopes recorded the fragments crashing into Jupiter's atmosphere.
Hale-Bopp	2400	1995	c. 4530	137 million	One of the brightest comets of the 20th century. Probably the most widely observed. It was visible to the naked eye for 18 months, and was brighter in the northern hemisphere than the southern. It is thought that its original orbital period was around 4200 years, but it passed close enough to Jupiter in 1995 that the planet's gravity considerably shortened the comet's period.
Encke	3.3	Aug 2000 *	Dec 2003 *	51 million	Encke has the shortest known period of any comet. Its period is decreasing by 2.5 hours on every orbit.

ooc: * information correct for current RPG date!

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