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Lecture 6, ASTA01

Chapter 2User’s Guide to the Sky:

Patterns and Cycles

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Stellar Coordinates

• Picture the Earth hanging at the centre of the celestial sphere. • Imagine the Earth's latitude and longitude

lines expanding outward and printing themselves onto the celestial sphere.

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Stellar Coordinates

• This provides a coordinate grid on the sky that tells the position of any star, just as latitude and longitude tell the position of any point on Earth. • In the sky, "latitude" is called declination and

"longitude" is called right ascension.

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Stellar Coordinates

• Declination is expressed in degrees, arc minutes, and arc seconds north (+) or south (-) of the celestial equator (just as latitudinal positions are described).• Capella, alpha Aurigae, the brightest star in

the constellation Auriga, has a declination of +46°0′ and is, therefore, about halfway between the celestial equator (dec = 0°) and the north celestial pole (dec = +90°).

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Stellar Coordinates: Sirius

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Stellar Coordinates

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Stellar Coordinates

If you orient the axis of the equatorial mount of a telescope toward the north celestial pole, you can fix the declination, turn the telescope or let it be turned by a clockwork, and it will follow the daily motion of a sky and a given star.

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Stellar Coordinates

• Right ascension (RA) is expressed not in degrees but in hours (h), minutes (m), and seconds (s) of time, from 0 to 24 hours.• Astronomers set up this arrangement long

ago because the Earth completes one turn in 24 hours, so the celestial sphere appears to take 24 hours to complete one turn around Earth.

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• The zero of right ascension is, by convention, the longitudinal line that runs through the spring equinox. • The RA for Capella is 5 hours 17 minutes east

of the spring equinox.

Stellar Coordinates

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Stellar Coordinates

• The celestial coordinates of stars remain constant over many years because they are so far away, even though the stars are moving relative to Earth. • On the other hand, the Sun, being much

closer, moves along the ecliptic throughout the year, travelling through the complete 24 hour right ascension zones over the calendar year.

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Timekeeping

• Even to the casual observer it is clear that the length of our day is related to the Earth's rotational period.• The number of days in a month must be

associated with the lunar phase period and the length of a year corresponds to the period of the Earth's orbit around the Sun.

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Timekeeping by Day

• Our local meridian is the imaginary line ending at the north and south celestial poles which cuts through our zenith. • The average length of time between

successive passes across the local meridian is called a solar day (this time varies slightly throughout a year which is why the word "average" is used).

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Timekeeping by Day

• Another way of determining the length of a day is to measure the time it takes for any star to make successive passes across the local meridian which we call a sidereal day (pronounced sy-deer-ial).

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Timekeeping by Day

• A sidereal day is about 23 hours 56 minutes, shorter than a solar day by about 4 minutes.• This is because during a solar day the Earth

has travelled along its orbit around the Sun and the Earth needs a little more time to rotate before the Sun crosses the meridian.

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• Timekeeping involving months comes from the lunar phase's cycle, which is about 29.5 solar days.• This roughly corresponds to the average

month length, known as a synodic month.• Synodic comes from the Latin word "synod"

meaning meeting – the meeting of the Sun and the Moon at each new moon phase.

Timekeeping by Month

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Timekeeping by Month

• If, however, we use the stars to measure the length of the lunar cycle, a sidereal month, the time turns out to be 27.3 days.• This is shorter than a synodic month for the

same reason a sidereal day is shorter than a solar day.

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Timekeeping by Year

• The length of a year is clearly related to the time required for the Earth to complete one full orbit around the Sun, about 365.26 days.• Again there are two slightly different

timeframes.

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Timekeeping by Year

• A sidereal year is the time taken for a complete orbit relative to the stars

• the time between successive spring (or autumnal) equinoxes is called a tropical year (or solar year) and it should come as no surprise that these two years differ.

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Timekeeping by Year

• A sidereal year is longer than a tropical year by about 20 minutes, the difference due to the precession of the Earth's rotation.• This 20-minute difference, if the sidereal year

was employed as a calendar, would result in seasons slowly shifting throughout the year, although this pattern would take 26 000 years to repeat.

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What Time Is It?

• We use an aspect of solar time. • The apparent solar time is determined by the

Sun's position in the sky relative to our local meridian.

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What Time Is It?

• When the Sun is right on the meridian it is noon.• Before the Sun gets to the meridian we say

that it is ante meridian (ante meaning before). • Hence a.m. or am.

• When the Sun has passed the meridian we say that it is post meridian (post meaning after).• Hence p.m. or pm

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What Time Is It?

• Each solar day differs from 24 hours by a slight amount because the Earth's orbit is not perfectly circular and because of the Earth's 23.5°tilt. • Thus, the average solar day is the more

important concept and the one used to keep track of time.

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What Time Is It?

• Using apparent solar time would mean adjusting clocks each day, an unnecessary complication.

• Also, apparent solar time varies with longitude (owing to the Earth's spin on its axis) and so everybody's apparent solar time will be different, unless they happen to be at precisely the same longitude.

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What Time Is It?

• To alleviate this problem Sandford Fleming, a Canadian, proposed a system of dividing the Earth into 24 different time zones such that within each time zone the time would be exactly the same. • Such a system was eventually adopted

universally by the late 1800s.

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Calendars

• The tropical year (equinox to equinox) is about 365.25 days. • If we choose 365 days for one year (the

Egyptian concept) then the seasons drift through the year by one day in every 4 years, not a great concept.• Julius Caesar introduced the idea that every four

years an extra day would be added to account for this discrepancy (hence the leap year), a definite improvement.

• This is the so-called Julian calendar.

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Calendars

• The tropical year is not exactly 365.25 days but rather about 11 minutes short of this value.• This results in the spring equinox moving

backwards through the calendar by 11 minutes each year, or about 14.5 hours/lifetime of 80 years, or about 12 days every 1600 years.

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Calendars

• In 1582, Pope Gregory XII introduced a slight variation in the calendar, which became known as the Gregorian calendar (the one we use today).• The Gregorian calendar first sets the spring

equinox to March 21 and then adjusts the leap day schedule.

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Calendars

• Each century year (normally a leap year) is skipped as a leap year unless that year is divisible by 400 (i.e., year 2000 is a leap year, but not 1900, nor 2100).• The Gregorian calendar is good for thousands

of years into the future and is now used globally.

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Night Sky Tours

• Many students take a university astronomy course to learn about the night sky, hoping to identify various star patterns and how they change throughout the year. • In this section, you will find a few “tours” of the

night sky visible to Canadian viewers that cover an entire year of observing.

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• Fortunately, you will not need a telescope – although a decent pair of binoculars may enhance your viewing experience. • Instead, you need a location with little or no

light pollution, an outdoor easy chair on which to lay to force your eyes upward (a blanket might be required depending on the weather), and your eyes!

Night Sky Tours

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Night Sky Tours

• In this section are typical star charts, identical to the ones found at the back of your textbook.

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Night Sky Tours

• Hold the chart with the words Southern Horizon at the bottom of the page, and be sure you are facing south. • The night sky will then appear above and

behind you as you move up the page. • The items indicated on the star chart to the

left (Eastern Horizon) will appear above you and to the left, and so on. • If you wish to begin by facing north, turn the star

chart upside down and follow the same procedure.

Night Sky Tours

Copyright © 2013 by Nelson Education Ltd.

• You will notice two arced lines stretching across the chart.

Night Sky Tours

• The yellow one, marked “Equator,” is the celestial equator, an imaginary circle around the celestial sphere, and is merely an extension of Earth’s equator.

Copyright © 2013 by Nelson Education Ltd.

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Night Sky Tours

• The orange arc, marked “Ecliptic,” is the line described earlier in this chapter that represents the path followed by the Sun across the sky once each year. • The 12 signs of the zodiac are found all along

the ecliptic

Night Sky Tours

• Now that you have a better understanding of star charts you are ready to take your first “tour” of the night sky.

Copyright © 2013 by Nelson Education Ltd.

Night Sky Tours

• To start, we will see what the sky looks like over a three- or four-month period.

Night Sky Tours

Night Sky ToursThe great summer triangle:DenebVegaAltair

Night Sky ToursThe great summer triangle:DenebVegaAltair

Summer triangle: Vega (alpha Lyr), Deneb (alpha Cyg), Altair (alpha Aql)

Summer triangle: Vega (α Lyr), Deneb (α Cyg), Altair (α Aql)

The stars of the Summer Triangle:

Vega (α Lyrae) mv=0.03 L=52 sp.type=A0 d= 25 lyDeneb (α Cygni) mv=1.25 L=70000 sp.type=A2 d=1550 lyAltair (α Aquilae) mv=0.77 L=10 sp.type=A7 d= 16.6 ly

Summer triangle: Vega (α Lyr), Deneb (α Cyg), Altair (α Aql)

shape of Vega dust disk Vega (α Lyrae) mv=0.03 L=52 sp.type=A0 d= 25 lyDeneb (α Cygni) mv=1.25 L=70000 sp.type=A2 d= 1550 lyAltair (α Aquilae) mv=0.77 L=10 sp.type=A7 d= 16.6 ly

Altair:first normal starto be imaged(“resolved”)

七夕 , Qī Xī (festival of sevens, 7th day of 7th monthe.g., Aug 23, 2012) (Tanabata in Japan)

Que Qiao)

織女 (Zhī Nŭ, English: Weaver Girl)

,النسر الطائر

牛郎星 ( Niú Láng Xīng)

Hikoboshi ( 彦星 )Cowherder star

Orihime ( 織姫 Weaving Princess)

Amanogawa ( 天の川 Milky Way, lit. "heavenly river”)

( 鹊桥 , "the bridge of magpies",

γαλαξίας κύκλος (galaxías kýklos)via lactea,

Flying eaglean-nasr aṭ-ṭā’ir

Orion

• Winter

constellation

Sirius

Aldebaran

Orion

Orion’s Belt

Orion’sNebula (M42)

in

Orion’s sword

Pleiades

Pleiades (M45, Seven Sisters) – open star cluster

Pleiades (M45, Seven Sisters) – open star cluster

Pleiades (10th century Anglo-Saxon illustration)

The Sky is now….

• Displayed on Google Sky http://www.google.com/sky/

• Zoomable down to regions a few arcminutes across• With overlayed constellations• Has overlayed infrared image showing dusty

hydrogen-helium clouds in the Galaxy• Has a catalog of Messier objects M1…M110 (for

instance, Andromeda galaxy is M31, Orion nebula M42 etc.)