Discussion

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Discussion
How do we estimate how far away things are from
us?
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Parallax
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Discussion
Does this work with astronomical objects, like
the stars, the Sun, the Moon and the planets?
Can you tell by looking at them which is further
from you than the others?
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Discussion
Can you tell by observing the Sun or Moon that
they are in fact closer to you than the stars?
If so, how?
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Occultation
Sometimes, the Moon can be seen to block the
light of a star as it passes in front of it.
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Occultation of Regulus
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Occultation of Saturn
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Discussion
Why does the Moon look much bigger near the
horizon than when it is high in the sky? Does it
get further away as it rises?
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Sun and Moon and relative size
When you see the Sun or Moon next to terrestrial
objects, your brain realizes that they are much
further away and registers them as much bigger.
When high in the sky, you have nothing to compare
it, so you see it with its true, smaller size.
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The dome of the sky
Because our depth perception does not work with
the stars, they all appear to be at the same
distance from us stuck on the inside surface of
a dome that surrounds us.
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Some terminology
Zenith the point in the sky directly over head.
Meridian an arc drawn from the north horizon
point to the south horizon point passing through
the zenith.
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Diurnal (daily) motion of the Sun
The Sun rises in the east and sets in the west.
Thus, each day it crosses the meridian. AM
ante meridian PM post meridian When the Sun
crosses the meridian it is also the farthest from
the horizon.
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Diurnal motion of the stars, Moon and planets
Just like the Sun, the stars, the Moon and the
planets rise in the east and set in the west,
each reaching its furthest point from the horizon
as it crosses the meridian.
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Circumpolar stars
Not all stars rise and set. Some stars, called
circumpolar, always appear above the horizon.
In fact, all the stars appear to circle a point
in the sky called the celestial pole. In the
north this point lies near the star Polaris, the
north star. In the south, the southern cross
points to this point in the sky.
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The celestial sphere
If we imagine the sky as the inside surface of a
sphere, we can specify any point on this sphere
using two coordinates. How do we measure
positions on the Earth?
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Latitude and longitude
On Earth we call these angles latitude and
longitude.
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The celestial poles and equator
The point in the sky directly above the Earths
north pole is called the north celestial pole.
Likewise, the point directly above the south pole
is called the south celestial pole. Midway
between the celestial poles (90 degrees away) is
the celestial equator, which lies directly above
the Earths equator.
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Discussion
How high above the north horizon point is the
north celestial pole as viewed from Earths north
pole? How high above the north horizon point
is the north celestial pole as viewed from
Earths equator? How high above the north
horizon point is the north celestial pole as
viewed from a latitude of 45 degrees north?
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Discussion
If Polaris appears 36 degrees above the horizon,
what is you latitude?
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Discussion
As viewed from a latitude of 42 degrees north,
how high above the southern horizon point is the
celestial equator?
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Positions in the sky
The angle from the celestial equator is called
the declination and the angle from the celestial
prime meridian is called the right ascension.
The celestial equator has a declination of 0
degrees, while the north celestial pole has a
declination of 90 degrees. The RA is measured in
hours where 360 degrees 24 hours.
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Earths rotation
The diurnal motion of the stars is caused by the
Earths rotation. Because the stars take 23
hours 56 minutes from one meridian crossing to
the next, this is Earths actual rotation period.
This is the rotation relative to the fixed
stars, or what we call the sidereal rotation
period.
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The annual motion of the Sun
The Sun on the other hand, takes an average of 24
hours between successive meridian crossings. The
difference is due to Earths revolution about the
Sun. The Sun moves on average 4 minutes
eastward each day relative to the stars, staying
in the sky 4 minutes longer each day than a star
at the same declination.
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Earths orbit speed is not constant
The Earth moves faster when it is closer to the
Sun (in January) and slower when it is further
away (in July). This means that the length of
the day is not a constant 24 hours during the
year. Instead of varying the length of the day
throughout the year we use the mean or average
position of the Sun to define our 24-hour day.
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Discussion
If the Earth moved faster in its orbit would the
length of the day be longer or shorter? Why?
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Annual motion of the stars
The same stars are not visible all year long.
Any given non-circumpolar star will set 4 minutes
early each day relative to the Sun, until it
becomes lost in the glare of the setting Sun.
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The ecliptic
The ecliptic is the annual path through the sky
that the Sun appears to take. In actuality,
the ecliptic is the plane of Earths orbit
projected onto the stars.
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The constellations
In modern astronomy, the constellations are 88
irregular areas that completely cover the sky.
Thus, every celestial object lies within the
boundaries of a constellation.
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Constellations of the Zodiac
The ecliptic passes through 12 constellation
during the year. This are know as the zodiacal
constellations.
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Discussion
In what zodiacal constellation is the Sun in
right now? What zodiacal constellation is
visible near the meridian at midnight?
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