#### Transcript Lecture06-ASTA01 - University of Toronto

```Lecture 6, ASTA01
Chapter 2
User’s Guide to the Sky:
Patterns and Cycles
1
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.
2
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.
3
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°).
4
Stellar Coordinates: Sirius
5
Stellar Coordinates
6
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.
7
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.
8
Stellar Coordinates
• 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.
9
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.
10
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.
11
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).
12
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).
13
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.
14
Timekeeping by Month
• 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.
15
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.
16
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.
17
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.
18
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.
19
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.
20
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
21
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.
22
What Time Is It?
• Using apparent solar time would mean
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.
23
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.
24
Calendars
• The tropical year (equinox to equinox) is
• 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.
25
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
every 1600 years.
26
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.
27
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.
28
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.
29
Night Sky Tours
• Fortunately, you will not need a telescope
– although a decent pair of binoculars may
• 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),
30
Night Sky Tours
• In this section are typical star charts,
identical to the ones found at the back of
31
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.
32
Night Sky Tours
• 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.
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
35
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.
Night Sky Tours
• To start, we will see what the sky looks
like over a three- or four-month period.
Night Sky Tours
The great summer triangle:
Deneb
Vega
Altair
Night Sky Tours
The great summer triangle:
Deneb
Vega
Altair
Night Sky Tours
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 ly
Deneb (α Cygni) mv=1.25 L=70000 sp.type=A2 d=1550 ly
Altair (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 ly
Deneb (α Cygni) mv=1.25 L=70000 sp.type=A2 d= 1550 ly
Altair (α Aquilae) mv=0.77 L=10
sp.type=A7 d= 16.6 ly
Altair:
first normal star
to be imaged
(“resolved”)

e.g., Aug 23, 2012)
(Tanabata in Japan)

Orihime (織姫 Weaving Princess)
γαλαξίας κύκλος (galaxías kýklos)
via lactea,
Amanogawa (天の川 Milky Way, lit.
"heavenly river”)
an-nasr aṭ-ṭā’ir
Flying eagle
‫الطائر النسر‬,
(鹊桥, "the bridge of magpies",
Que Qiao)

Hikoboshi (彦星)
Cowherder star
Orion
• Winter
constellation
Aldebaran
Sirius
Orion
Orion’s Belt
Orion’s
Nebula (M42)
in
Orion’s
sword
Pleiades (M45, Seven Sisters) – open star cluster
Pleiades (M45, Seven Sisters) – open star cluster