astro 6 Flashcards
The tropical year is about 20 min. shorter than
the sidereal year
A calendar based on the sidereal year gets out of synch with the seasons by
1day every 72 years – a difference that adds up over the centuries.
The difference between the sidereal year and the tropical one arises from
Earth’s ~26,000-year cycle of precession, which
changes not only the orientation of
Earth’s axis in space, but also changes
the locations in Earth’s orbit at which the
seasons occur
Synodic Month
The cycle of lunar phases of about 29.5 days, 1/12 longer than a sidereal month
The reason for the difference between the synodic & sidereal months is that
just as a solar day is not Earth’s true rotation period, a synodic month is not the Moon’s true orbital period. Earth’s motion around the
Sun means that Moon must complete more than one full orbit of Earth
from one new Moon to the next.
Like the sidereal day, the
sidereal month gets its name
because it describes how long it
takes the Moon to complete an orbit relative to the position of distant stars.
Because Earth rotates at the same time it orbits the Sun
it needs to
make up for the orbital motion by making slightly more than a full
rotation around its axis
This extra bit of rotation makes a solar day longer than a sidereal day!
Sidereal day = the time for Earth to rotate once on its axis =
= 23 hrs, 56 min, and 4.07 sec.
From our perspective, the Sun
moves about
1 degree from W to E with
respect to the ‘fixed’ stars.
While the Earth is rotating on its axis it is also moving along its orbit around the Sun
Penumbral lunar eclipse
–
Moon only passes through
penumbra
Partial lunar eclipse
part of full moon passes through umbra
Total lunar eclipse –
– Moon passes entirely through umbra
Lunar eclipse begins
when
the Moon enters Earth’s penumbra.
After that, one of the 3
types of lunar eclipse can
be seen:
Penumbral lunar eclipse –
Partial lunar eclipse –
Total lunar eclipse –
There are three types of solar
eclipses:
Total solar eclipse
Partial solar eclipse
Annular solar eclipse
Umbra =
sunlight is completely blocked.
Penumbra =
= sunlight is partially blocked; surrounds umbra
Azimuth:
angle direction along horizon, clockwise from due North
Altitude:
angle above the horizon
The Sun moves randomly relative to other nearby stars at typical speed of more than
70,000 km/h
he Sun orbits
the galactic
center once
every
230
m
years at a speed
of 800,000 km/h
Perihelion:
The nearest
point to the
Sun in orbit
Aphelion:
The farthest
point from
the Sun in
orbit
Earth’s average orbital speed
around the Sun
108,000 km/hr
Earth’s orbital path defines a flat plane called the
ecliptic plane.
Pythagoras:
Introduced the concept of “number” as truth in mathematics that allowed for an objective comprehension of reality He & his followers envisioned Earth as a sphere at the center of the celestial sphere
Parallax =
= apparent shift, back & forth, of nearby stars against the background
(“fixed”, distant stars).
Greeks concluded that only
one of the following must be true:
Earth orbits the Sun but the stars are so far
away that stellar parallax is undetectable to the naked eye, OR
There is no stellar parallax because Earth
is stationary at the centre of the Universe.
Ancient Greeks rejected the correct answer
because they could not believe that the stars
could be that far away.
We will revisit stellar parallax later on
Ptolemy perfected the Geocentrical Model
Applied an idea from Apollonius: each planet moved on a small circle
(‘epicycle’) which orbited around Earth on a larger circle (‘deferent’)
He also relied heavily on the work of Hipparchus who had developed
Apollonius’s model by adding several features
Galileo s Principle of Relativity:
“It is impossible by mechanical means to say whether we are moving or staying at rest”.
Galileo’s rolling balls experiments showed that a
a moving object remains in motion unless a force acts to stop it (Newton’s 1st Law)
Ptolemaic Model:
Only new & crescent phase
Heliocentric model:
All phases (like our Moon)
p
mv
Built the 1st reflecting telescope
newton
From Earth’s surface, escape velocity is ~
~11 km/s, i.e. ~40,000 km/h
Orbital energy
kinetic energy + gravitational potential energy = ct.!
Spring tides:
when the Sun &
Moon work together during new& full Moon.
Neap tides:
when the tidal forces from the Sun & Moon
counteract each other during first- & third-quarter Moon.
mercury high density and small size
most probably suffered a
huge impact in its “youth” that blasted (most of) its outer
layers away.
why does mercury have no moons
it orbits too close
to the Sun which will take it
away if it had any.
Has a magnetic field with a strength ~1% that of Earth’s
mercury
Quantum theory of light
: Energy radiated or absorbed can not have
any fractional value. This energy must be an integral multiple of a
fixed quantity of energy called “QUANTUM”.
PHOTONS
particles that have no (rest) mass & no charge
e =
hv
Louis de Broglie
consolidated the
concept of dual nature by enquiring: “Given that light behaves as waves
and particles, can particles of matter behave as waves?“. He predicted
that all matter exhibits wavelike motions by proving that there are
particles (electrons =e–, protons=p+ and neutrons) besides photons that have the properties of a wave.
nucleus is nearly
100,000 times smaller than the atom
electrons are
smeared out in a cloud around the nucleus
what could fit end to end across this dot
ten million atoms
A very hot gas in which all atoms have been ionized is
plasma
A molecule contains
electronic, vibrational, and rotational energy levels.
Each electronic level is related to a number of vibrational levels with less
energy separation, and every vibrational level is related to many rotational
levels with even less energy separation.
The vibrational transitions result in deformations of the bonds between the
various atoms within the molecule.
Very important as it allows the identification of many compounds using
Fourier Transform Infrared Spectroscopy (FTIR)
Four ways in which light can interact with matter:
emission, absorption, transmission, reflection, scattering
Reflection =
macroscopic; at a surface;
Scattering =
= at micro scale; within the bulk of a material, due to
its constituent particles.
Composition of the Sun (and stars in general) is deduced from
spectroscopic measurements of their emitted light
Hence, spectroscopy is
s the decomposition of an object’s
light (i.e. dispersion) into its component ‘colors’ (i.e.
wavelengths) which are subsequently measured. From this analysis of an object’s light, astronomers can
infer the physical properties of that object (such
as temperature, mass, luminosity and composition).
How can we do this for visible light?
Remember that different wavelengths in the visible range correspond
to different colors
(Visible) Light can be dispersed using a prism, or a diffraction grating
Spectroscopy can be used to derive many properties of:
Distant stars and galaxies, such as their chemical composition,
temperature, density, mass, distance, luminosity, and relative motion
using Doppler shift measurements.
Planets (their surface or atmosphere composition);
Other astronomical objects (comets, cosmic dust clouds, etc.)
In the 1850s Gustav Kirchhoff & Robert Bunsen first showed that
Hot solid objects produce light with a continuous spectrum,
Hot gasses emit light at specific wavelengths (emission lines), and
Hot solid objects surrounded by cooler gases will show a near continuous spectrum with dark lines corresponding to the emission
lines of the gasses.
origin of emission and absorption lines
Quantum structure of the atom, and
Blackbody radiation emission
The spectrometer
decomposes incoming light and accurately measures the intensity I of each wavelength λ, providing the I(λ) plot, i.e spectrum of (absorbed, or transmitted, or reflected) light.
Thermal radiation is also known as
blackbody radiation
Stars behave
like a
blackbody
Blackbody =
a theoretical object:
Blackbody =
• It is
a perfect absorber for all incident radiation.
• It also is an ideal diffuse (isotropic) emitter
Refracting telescopes
use lenses
to collect & focus light
Reflecting telescopes
s use curved
mirrors to collect & focus light
(used exclusively in professional
astronomy today)
The refractive telescope:
uses lenses to
concentrate light
The reflective telescope:
relies on mirrors to
reflect & focus light
What do astronomers do with the telescope
Imaging = taking pictures Using filters, image processing and combining more images in a single one can reveal new details and enable better understanding of astronomical phenomena Timing = study how an object changes over time Spectroscopy = gather information from the object’s spectrum (separate different wavelengths before they hit the detector)
The spectral lines are due to
the
unique set of discrete energy levels for each type of atom, ion or molecule Unique spectral “fingerprint” that can be used to identify the chemicals in celestial objects
