1. Astronomy is the study of the origin, evolution, composition, distance and motion of objects in the universe. 2. Early astronomy was used for navigation, timekeeping, and predicting seasons important for agriculture. 3. The solar system includes eight planets, thousands of asteroids, comets, and meteoroids orbiting the sun.

1. ELEMENTARY ASTRONOMY
Introduction
The word astronomyis derived from the Greek astron, meaning "star"
and nomos, meaning "laws or cultures". Theliteralmeaning ofthe
word astronomy thereforeis "law of the stars".
Astronomy is thebranch of science that dealswith study of the origin,
evolution, composition, distanceand the motionof all bodies(objects) and
scattered matterinthe universe.
The universe is the totalityof spaceand timetogether with matter and
energy.
People who are involved in astronomyareknown as astronomers.
Astronomersstudythe objectsthat composethephysicaluniverse, stars,
planets, moons, galaxiesand nebulae.
Astronomyis one of the oldest fields of science. Early astronomyinvolved
observing the regular patternsof the motionsof visible celestialobjects,
especially, thesun, moon, starsand planetswith their naked eyes. An
exampleof astronomywasthe study of the changing positionofthe sun
along the horizon or the changing appearancesof starsin the courseof the
year. These were used to establish agriculturaland ritualcalendars.
Importance of astronomy to mankind
Astronomyhas been an important toolfor thousandsof years. The
following are some ways in which astronomyis important;
1. It was the earliest method of measuring time. A daywas the duration
betweensunrise and sunset while a month wasderived from phases
of themoon. The year was derived from the changing positionof
sunrise.
2. It was used to develop calendarsthat madeit possibleto predict the
seasons. The season was very importantinagricultureasthey
dictated theplanting timeand theharvesting time.
3. It was used in both land and sea navigationbased on theknowledge
of thepositionof the sun during the dayand the stars at night.

2. 4. Today, astronomyhelps us to understand wherethe earth and the life
it supportsoriginated from and how it evolved.
5. Astronomypresents a new frontier for exploration.
The solar system
The solar system is madeup of the sun arid thecelestial objectsbound to it
by gravity. The objectsincludethe eight planets and their known moons
and billionsof small bodies that includeasteroids, comets, meteoroidsand
interplanetarydust.
Solar system
Stars and planets
A star is a large celestialbody madeup of hot as known as plasma.
Plasma refers to an ionized gas in which a certainproportionof electrons
are free rather thanbound to an atom or molecule. Starsradiateenergy
derived from the thermonuclear reactionsinthe interior region. Thesun is
the largest star.
The sun is also the closest star to the earth at a meandistanceof 149.60
million kilometers. Thisdistanceisknown as the astronomical unit
and is used to measuredistancesacrossthesolar system.

3. Structure of a star - the sun
A galaxy is a giant collectionof stars, gasand dust. Most starsin the
universe arein the galaxies. Nearlyall of the starsvisible in the night sky
are withinour own galaxy, our galaxyis called the Milky way galaxy.
A planet isa major (large) object which is in orbit around a star. There are
eight such objectswhich arein orbit around thesun. These arecommonly
referred to as "the planets". They are Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranusand Neptune.
The following are the three defining characteristics ofa planet:
1. It is a celestialbody that orbitsa star.
2. It is massive enough so that itsown gravitycausesit to assumea
sphericalshape.
3. It has cleared the neighborhood around itsorbit.
NOTE:BecausePlutoresidesin an area of spacepopulated by
numerousother objects, it is no longer considered a planet. Plutois
now designated a dwarfplanet. A dwarfplanet does not meet the
third characteristic, i.e. has not cleared the neighborhood around its

4. orbit.
Differences between stars and
planets
stars Planets
Emit own light. Do not emit their own light.
Twinkle at night. Do not twinkleat night.
Appear tobe moving from east to
west.
Planetsmove around the sun from
west to east.
Their temperaturesareusuallyvery
high.
Their temperaturesdepend on their
distancesfrom the sun.
Countless in number. There areeight in the solar system.
Very big in size but they appear
small becausethey arevery far
away.
Very small in size as compared tostars.
Asteroids, sometimescalled minor planets, aresmallsolar system
bodiesin orbit around the sun, especially in the inner Solar System.
Asteroidsaresmaller thanplanets but larger thana speck of dust.
A comet is a solid body orbiting thesun typicallycomposed of rock dust,
or ice. Most cometswere formed from condensed interstellar gasand dust
clouds in the early stagesof thecreationof the universe.
Gravitational force
The termsgravitationalforceand gravityareused often to explainthe
something, but thereis a definitedifferencebetweenthe two terms.
Gravitational force is the attractiveforceexisting betweenanytwo
objectsthat havemass. Gravitationalforcepulls objectstogether.
Sincegravitationalforceactson all matter inthe universe, from the largest
starsto the smallest atoms, it is often called universal gravitation. Sir
Isaac Newtonwas the first person to fully recognizethat the force holding
any object to theearth is the same as the forceholding the moon, the

5. planets, and other heavenly bodies in their orbits.
According toNewton'slaw of universal gravitation, everysingle point mass
attractsevery other point mass by a force directed along theline joining the
two masses. The force is proportionaltothe product of the two massesand
inversely proportionaltothe squareof the distancebetweenthepoint
masses:
F=GM1 M2
r²
Where F is themagnitudeofthe attractivegravitationalforcebetweenthe
two point masses; G is the universal gravitationconstant; m1isthemass of
the first point mass; m2 is the massof the second point mass; and r is the
distancebetweenthecenters of the two point masses.
Gravitationalforceisactuallya very weakforce. The pull is too weak to be
felt between twopeople. It is only when one of the massesis very largeto
the size of a planet that the force of gravitycanbe felt.
The huge gravitationalforceof thenearest star, the sun, holds together the
eight planetsof the solar system. The planetsmove through spaceat speeds
that just balancethesuns gravitationalpull, so they arelocked into a
permanent path (orbit) around thesun. Naturalsatellites(moons) orbit
planets. Artificial(humanmade) satellitesand spacecraftorbit theearth' in
the sameway as the moon orbitsthe planets.
Gravitydenotesthe gravitationalforcethat occursbetweentheearth and
other bodies. Gravityisthe force acting topull objectstowardstheearth.
Gravityis theforce that holds us on the ground and causesobjectsto fall
backto the ground after being thrownup in the air.
The earth'sgravitationalpullextendsout into spacein all directions. The
further you move awayfrom, the center of theearth, the weaker theforce
becomes.
The measureof the force of gravityon an object on the earth'ssurfaceis the
weight of that object Weight ismeasured in Newton’s (N). The weight of
an object changesdepending on its locationin the universe.

6. Constellations
A constellation: isa group of starsthat form a definiteshapeor pattern
when viewed from the earth.
Constellationsare usually named after mythologicalcharacters, people,
animalsand things.
There areabout 88 known constellations. The variousconstellationsare
visible during a particularperiod of the year.
Some examples of the known constellationsare shown, in the
table below;
Uses of constellations
The following are some uses of constellations:
1. Religious
In early days, people thought that thegods lived in the heavens and that the
gods created theconstellations. Manyculturesbelieved that the positionsof
the starswere their god's way of telling stories. Indeed, the Greeksnamed
the constellationsafter their mythologicalheroesand legends. For example,
to the ancient Greeks, Orion was a great hunter. He was the son of Neptune
(god of the sea).
2. Agricultural

7. Before therewere proper calendars, people had no way of determining
when to sow or harvest except by the stars. Constellationsmadethe
patternsof the starseasy to remember. Theancient peoples knew, for
example, that when the constellationOrionstarted tobe fully visible,
winter was coming soon. The constellationsallowed farmersto plan ahead.
3. Navigation
It is fairly easy to spot Polaris(The North Star) onceyou have found Ursa
Minor (Little Dipper constellation). One canfigureout his or her latitude
(North or South) just by looking at how high Polarisappearsin the night
sky. Thisallowed for shipsto travel acrossthe globe. It allowed for the
discoveryof America, thespread of European cultureand civilizationaswe
know it today.
The earth and its moon form a unique pair in the solar system.
The moon of the earth is the sixth largest in thesolar system. It hasa
diameter of 3476km and a massof 7.35 x 1022 kg.
Besides the earth, the moon is the only other body in the solar system upon
which humankind haswalked.
Astronaut Buzz Adrian on the moon with the Apollo 11
spacecraft.

8. Like the earth, the moon has an iron coresurrounded by a rocky mantle
and crust. Unlike the earth, no part of the moon's iron coreis molten so it
does not have a magnetic field. Surfacegravityonthe moon is about 1/6
that on the earth. An object weighing 120kg on the earth would only
weight 20 kg on the moon.
The moon revolves in an anticlockwisedirectionaround theearth in an
elliptical orbit. Themoon's orbit is tilted at 5° relativeto the earth'sorbit
around the sun. The distancebetweenthe earth and the moon varies from
perigee(nearest the earth) where it is 356,000km to apogee(furthest
from the earth), whereit is 406,000km. The averagedistanceis384 000
km. It takes the moon 27.3 earth days to completeone orbit, a period of
timecalled the Siderealmonth.
The moon also rotatesabout itsaxisat a rateequal to itsrateof revolution.
In other words while the moon is completing one orbit around theearth, it
also spins round one time. Theresult is that the sameside of the moon is
always facing theearth. The side facing theearth is called the near side
while the side that facesawayis called the far side. The spinning of the
earth causesthe moon to rise and set each day, just like the sun. However,
becauseof the moon's orbitalmotionaround the earth, it (the moon) rises
about 50 minuteslater each day. As a result, themoon canbe seen at
different timesof the dayand night during a month.
Temperaturesonthe surfaceof the moon are on average 107°C during the
day and -53°C during the night.

9. Surface features of the moon
There aretwo primarytypesof terrainonthe moon. These are the heavily
cratered very old lunar highlands and therelatively smooth and Younger
Maria.
From the earth, the moon's surfaceappearstohave bright and darkregions
when viewed with the unaided eye. The bright areasarethe lunar highlands
that have manycratersand are covered with a highlyreflective layer of fine
dust. Thehighlandsare geologicallythe oldest partsof themoon's surface.
The darkregions arelow areassimilar toocean basinson the earth. They
are filled with darksolidified lava and are less cratered thanthehighlands.
Galileo called these areas Maria, the Italianword for seas, becausetheir
darksmooth surfacesappeared tobe largebodies of water.
A photograph ofthe moon's surface showing highlands and
Maria.
The Maria, which makesabout 16% of themoon's surface, arehuge impact
cratersthat were later flooded with molten lava. Most of the Maria {s
covered with regolith, a mixtureoffine dust and rocky debrisproduced by
meteor impact.
Ocean tides
Tidesare periodic risesand falls of largebodies of water. Tidesare caused
by the gravitational interaction betweentheearth and the moon.

10. The earth and themoon areattracted toeach other, just like magnetsare
attracted toeach other. The moon triesto pull at anything on the earth to
bring it closer. But the earth is able to hold onto everything, except the
water. Sincethe water is alwaysmoving, theearth cannot hold onto it and
the moon is able to pull at it. This results in oceantides. Each day, thereare
two high tideand two low tides.
The oceanis constantlymoving from high tideto low tide, and thenbackto
high tide. There is a timeintervalof about 12 hours and 25 minutes
betweenthe two high tides.
How tides occur
The gravitationalattractionofthe moon causes the oceansto bulge out in
the directionofthe moon. Another bulgeoccurs on the oppositesidesince
the earth is also being pulled towardsthemoon (and awayfrom the water
on thefar side). Oceanlevels fluctuatedailyas the sun, moon and earth
interact. Asthemoon travelsaround the earth and as they together travel
around the sun, the combined gravitationalforcescausethe world ocean
water levels to rise and fall Sincethe earth is rotating whilethisis
happening, twotidesoccur each day.
Types of tides
There aretwo maintypes of tides. These are spring tides and neap tides.
Spring tides

11. Spring tidesoccur during thefull moon and the new moon. During this
time, the earth, the sun and themoon arein a line. Thegravitationalforces
of themoon and the sun both contributetothetides. At these times, the
high tidesare very high and thelow tidesare very low. These are known as
a spring high tideand a spring low tide, respectively. Spring tidesare
especially strong tides.
Proxigean spring tide is a. rareunusually high tide. The Proxigean
spring .tideoccurswhen the moon is both unusually close to the earth (at
itsclosest perigee, called the-proxigee) and in thenew moon phase' (when
the moon is betweenthe sun and the earth). The proxigeanspring tide
occursat most once every 1.5 year.
Neap tides
When the sun and themoon arenot aligned, the gravitationalforcescancel
each other out, and the tidesare not.
Very high or very low, these are called neap tides.
Neap tidesoccur during quarter moons. During thistime, the gravitational
forces of themoon and the sun are perpendicular toone another (with
respect to the earth). Thiscausesthe bulges to canceleach other. The result

12. is a smaller differencebetweenhigh and low tideand is known as a neap
tide. Neap tidesare especially weaktides.
Chapter summary
1. Astronomyis the study of the originevolution, composition, distance
and the motionof all objectsand scattered matterinthe universe.
2. In early times, astronomywas used to measuretime, predict seasons
and in navigation.
3. The solar system is madeup of the eight planets, the sun, thousands
of asteroidsand countless cometsand meteoroids.
4. Starsaregiant spheresof hot gases called plasma.
5. A galaxyisa large group of stars, dust and gasheld together by
mutualgravitationalforces.
6. Gravitationalforceisthe attractiveforcethat existsbetweenany two
objectsthat havemass.
7. Gravityis thegravitationalforcethat existsbetweentheearth and
other bodies.
8. A constellationis a group of starsthat form a definitepatterninthe
sky when viewed from theearth.
9. Constellationsare used in religion, agricultureand innavigation.
10. The earth has one moon. Themoon of the earth is the sixth largest in
the solar system.
11. Interactionbetweenthesun, the earth the moon and the oceanwaters
results in oceantides.
12. There aretwo maintypes of ocean tides. These are spring tidesand
neap tides.