introduction to astronomy - open...

Introduction to Astronomy

Lecture 3: The Sun-Earth-Moon System

Moon Phases, Tides, and Calendars

1 Elizabeth Charlton, 2015

The Sky from Earth


The Celestial Sphere

•  For objects such as the Sun and stars, we cannot directly perceive their distance

•  Historically the sky was perceived as a sphere with little lights on it

Earth within celestial sphere" by Tfr000 (talk) 20:06, 29 March 2012 (UTC) - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Earth_within_celestial_sphere.gif#mediaviewer/File:Earth_within_celestial_sphere.gif


The ecliptic

Elizabeth Charlton, 2015 4

•  The apparent eastward path of the Sun throughout the year –  the plane of the

Earth’s orbit –  tilted with respect to

the celestial equator due to the tilt of the Earth’s axis (23.5o)

"Ecliptic with earth and sun animation" by Tfr000 (talk) 16:54, 15 March 2012 (UTC) - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Ecliptic_with_earth_and_sun_animation.gif#mediaviewer/File:Ecliptic_with_earth_and_sun_animation.gif

The ecliptic and the celestial sphere

•  Equinox – point where the ecliptic crosses the celestial equator

•  Solstice – point where the ecliptic and celestial equator are at the maximum 23.5o separation


"Earths orbit and ecliptic" by Tfr000 (talk) 01:59, 15 March 2012 (UTC) - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Earths_orbit_and_ecliptic.PNG#mediaviewer/File:Earths_orbit_and_ecliptic.PNG

Coordinates

•  Just like the curved surface of the earth has a coordinate system …

•  So does the sky, 2 systems in fact – Altitude and Azimuth – Right Ascension and Declination


Altitude-Azimuth Coordinates •  Also known as the Horizontal coordinate

system – The observer’s local horizon is the fundamental

plane separating the hemisphere’s – Altitude – or elevation. The angle between the

object and the local horizon – Azimuth – the angle of the object around the

horizon – Zenith – the point directly overhead – Zenith Distance – the distance from zenith


Altitude-Azimuth Coordinates


"Azimuth-Altitude schematic" by TWCarlson - http://commons.wikimedia.org/wiki/File:Azimut_altitude.svg. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Azimuth-Altitude_schematic.svg#mediaviewer/File:Azimuth-Altitude_schematic.svg

Equatorial Coordinates •  Celestial Coordinate System – Analogous to

the Longitude-Latitude Coordinate System on Earth – The projection of the Earth’s equator onto the

Celestial Sphere is the fundamental plane (units: degrees arcsec arcmin)

– Right Ascension – Measures the angular distance of an object eastward along the celestial equator from the vernal equinox (units: hr min sec)

– Declination – Measures the angular distance of an object perpendicular to the celestial equator


Equatorial Coordinates


"Ra and dec demo animation small" by Tfr000 (talk) 20:50, 17 April 2012 (UTC) - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Ra_and_dec_demo_animation_small.gif#mediaviewer/File:Ra_and_dec_demo_animation_small.gif

Cycles of the Sky


Motion of the Sun and Stars

•  The observed motion of the Sun and Stars is due to the motions of the Earth – rotation, revolution, precession

"AxialTiltObliquity" by Dna-webmaster - self-made by Dna-webmaster; earth-image from NASA. Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:AxialTiltObliquity.png#mediaviewer/File:AxialTiltObliquity.png


Motion of the Sun

•  Observing from the Earth, how does the Sun move? – Daily Motion – Yearly Motion

Copyright © James B. Kaler. All rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's express consent except in fair use for educational purposes. Thanks to reader number counter on blogger. 13 Elizabeth Charlton, 2015

Motion of the Sun

•  Daily Motion – What do we observe? – Due to the rotation of the Earth – Day – 86,400 SI seconds (24 hours) is the

Astronomical unit of time – Astronomical Definitions

•  Mean Solar Day – average time of a single rotation of the planet with respect to the Sun (average over the year = 24 hours)

•  Sidereal Day – an entire rotation of the planet with respect to the stars (3min 56sec shorter than solar day)


Solar vs Sidereal Day

•  Sidereal Day is shorter than the Solar Day


"Sidereal day (prograde)". Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Sidereal_day_(prograde).png#mediaviewer/File:Sidereal_day_(prograde).png

1 to 2 is one sidereal day

1 to 3 is one solar day

Motion of the Sun

•  Yearly Motion – What do we observe? – Due to the revolution of the Earth – Year – A Julian year is defined as exactly

365.25 days of 86,400 SI seconds (24 hours). – Other Astronomical Years

•  Sidereal Year – the time it takes the Earth to complete one revolution with respect to the fixed stars (fixed frame of reference)

•  Tropical year – the time it takes the Sun to return to the same position in the cycle of the seasons (for example: vernal equinox to vernal equinox)


Keeping Time

•  Using the position of the Sun or stars to keep time – Astrolabes – Sundials


"Iranian Astrolabe 14" by Jacopo Koushan (User:Jacopo188)Photograph by Masoud Safarniya (User:M.safarniya) - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Iranian_Astrolabe_14.jpg#mediaviewer/File:Iranian_Astrolabe_14.jpg

"Equatorial sundial topview" by WillowW - Own work. Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Equatorial_sundial_topview.gif#mediaviewer/File:Equatorial_sundial_topview.gif

Keeping Time •  At local noon the sun is at its highest point in the sky

(crosses the local meridian) •  This is different for every longitude

–  Time Zones – Offset from Universal Time


"World Time Zones Map" by TimeZonesBoy - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:World_Time_Zones_Map.png#mediaviewer/File:World_Time_Zones_Map.png

"Meridian on celestial sphere" by Tfr000 (talk) 16:56, 18 June 2012 (UTC) - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Meridian_on_celestial_sphere.png#/media/File:Meridian_on_celestial_sphere.png

Motion of the Sun

•  How does the motion of the sun depend on the time of year?

•  How does the motion of the sun depend on the location of the observer?


Day Arcs

•  At the equator – path during the solstices


"Solstice-0" by Tauʻolunga - Own work. Licensed under Creative Commons Attribution-Share Alike 2.5 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Solstice-0.jpg#mediaviewer/File:Solstice-0.jpg

Day Arcs

•  At 20o latitude – path during the solstices



Day Arcs


Elizabeth Charlton, 2015 22 "Solstice-50" by Tauʻolunga - Own work. Licensed under Creative Commons Attribution-Share Alike 2.5 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Solstice-50.jpg#mediaviewer/File:Solstice-50.jpg

Day Arcs


Elizabeth Charlton, 2015 23 "Solstice-70" by Tauʻolunga - Own work. Licensed under Creative Commons Attribution-Share Alike 2.5 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Solstice-70.jpg#mediaviewer/File:Solstice-70.jpg

Day Arcs




Precession

•  Precession – change in the orientation of a rotating body –  Axial precession of the

Earth – movement of the rotational axis

•  Complete precessional cycle is 26,000 years or 1o every 72 years

•  Also called precession of the equinoxes


"Earth precession" by NASA, Mysid - Vectorized by Mysid in Inkscape after a NASA Earth Observatory image in Milutin Milankovitch Precession.. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Earth_precession.svg#mediaviewer/File:Earth_precession.svg

Precesssion


"Precession N" by Tauʻolunga - self, 4 bit GIF. Licensed under Creative Commons Attribution-Share Alike 2.5 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Precession_N.gif#mediaviewer/File:Precession_N.gif

Calendars

•  A way of organizing time – periods are usually synchronized with the cycle of the Sun or Moon.

•  Calendars in use today: –  Gregorian Calendar –  Islamic Calendar –  Hindu Calendars


"Hindu calendar 1871-72" by Unknown - http://www.loc.gov/exhibits/world/images/s92.jpg Description: http://www.loc.gov/exhibits/world/world-object.html. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Hindu_calendar_1871-72.jpg#mediaviewer/File:Hindu_calendar_1871-72.jpg

Gregorian Calendar •  International Standard

–  refinement to the Julian Calendar in 1582 by the Catholic Church

–  also reformed the lunar calendar used by the church –  corrected the length of the year by .002% –  brought the date of Easter to the correct time of year

(agreed upon by the First Council of Nicaea in 325) •  Leap years

–  a year containing an extra day to keep the calendar year synchronized with the astronomical or seasonal year

–  most years that are integer multiples of 4 are leap years –  the century years must be integer multiples of both 100

and 400


Motion of the Stars

•  Diurnal Motion – the apparent daily motion of stars around the celestial poles – due to the rotation of the Earth – visible stars depend on your location on the

Earth and where the Earth is in its orbit

"Deep Crux wide field with fog" by Naskies - Own workPreviously published: http://www.iceinspace.com.au/forum/showthread.php?t=89854. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Deep_Crux_wide_field_with_fog.jpg#mediaviewer/File:Deep_Crux_wide_field_with_fog.jpg 29 Elizabeth Charlton, 2015

The Reasons For Seasons

What do you think?


Hypothesis #1

•  The energy put out by the sun changes over the year

•  Prediction: – The energy should be measurably different

from space •  Interpretation:

– Hypothesis is wrong (energy doesn’t vary)


Hypothesis #2

•  The distance from the sun changes •  Prediction:

– The distance should be measurably different –  It should be warmest when closest

•  Interpretation: – Hypothesis is wrong (coldest when closest)


"Seasons1" by following Duoduoduo's advice, vector image: Gothika. - [1]. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Seasons1.svg#mediaviewer/File:Seasons1.svg

Hypothesis #3

•  The angle to the sun changes •  Prediction:

– The angle should be measurably different –  It should be warmest when sun must direct – Seasons should be opposite in southern

hemisphere •  Interpretation:

– Hypothesis is right


"North season" by Tauʻolunga - Own work. Licensed under Creative Commons Zero, Public Domain Dedication via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:North_season.jpg#mediaviewer/File:North_season.jpg

"Earth-lighting-summer-solstice EN". Licensed under Creative Commons Attribution-Share Alike 2.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Earth-lighting-summer-solstice_EN.png#mediaviewer/File:Earth-lighting-summer-solstice_EN.png

"Earth-lighting-winter-solstice EN". Licensed under Creative Commons Attribution-Share Alike 2.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Earth-lighting-winter-solstice_EN.png#mediaviewer/File:Earth-lighting-winter-solstice_EN.png

"Seasonearth". Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Seasonearth.png#mediaviewer/File:Seasonearth.png

Phases of the Moon

!  The appearance changes !  Amount of illumination !  Location of illumination

!  What are the phases?

"Lunar libration with phase Oct 2007 450px" by Tomruen - Lunar_libration_with_phase_Oct_2007.gif. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Lunar_libration_with_phase_Oct_2007_450px.gif#mediaviewer/File:Lunar_libration_with_phase_Oct_2007_450px.gif


Phases

!  Name !  New !  Crescent !  Quarter !  Gibbous !  Full

!  % illuminated !  0 % illuminated !  0-50 % illuminated !  50 % illuminated !  50-100 % illuminated !  100 % illuminated


Waxing verses Waning

!  Waxing means !  % illuminated is

increasing !  Waning means

!  % illuminated is decreasing


When is the Moon visible?

•  The moon is not always up at the same time.

•  The Moon rises roughly 50 minutes later each day

•  Sometimes up at night •  Sometimes up during the day


Relative Positions


"Moon phases en" by Orion 8 - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Moon_phases_en.jpg#/media/File:Moon_phases_en.jpg

Tides •  What are tides?

–  The regular rise and fall of the water level surface

•  What causes tides? –  The gravitational

interaction between the Earth and Moon

–  The gravitation of the sun also contributes to a smaller extent "Tide schematic". Licensed under Public domain via Wikimedia Commons - http://

commons.wikimedia.org/wiki/File:Tide_schematic.svg#mediaviewer/File:Tide_schematic.svg


"Tide overview" by iThe source code of this SVG is valid.This vector image was created with Inkscape. - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Tide_overview.svg#mediaviewer/File:Tide_overview.svg


Timing of tides

•  How often is there a high tide? – Approximately every 12 hours

•  How often is there a low tide? – Approximately every 12 hours

•  Why do the times of high and low tide change from day to day? – The moon is also orbiting around the Earth

each month


Eclipses

•  Two types of eclipses:

– Solar

– Lunar

"Eclipse lune" by Luc Viatour - own work www.lucnix.beNikon case FE2 Otical refractor Vixen 1000mm F/D10addition of several images in Photoshop. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Eclipse_lune.jpg#mediaviewer/File:Eclipse_lune.jpg

"Solar eclipse 1999 4 NR" by Luc Viatour - Own work www.lucnix.be. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Solar_eclipse_1999_4_NR.jpg#mediaviewer/File:Solar_eclipse_1999_4_NR.jpg


Solar eclipse

!  Definition…

!  The shadow of the moon falls on the Earth


Eclipse as seen from space

Credit: Mir 27 Crew; Copyright: CNES 49 Elizabeth Charlton, 2015

As seen from Earth

!  Moon blocks out sun


Solar Eclipse Geometry

!  Where does the moon need to be compared to the earth and sun for a solar eclipse to happen?

!  Moon is between Sun and Earth !  Possible at new moon phase


Solar eclipse

"Geometry of a Total Solar Eclipse" by Sagredo - Own work. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Geometry_of_a_Total_Solar_Eclipse.svg#mediaviewer/File:Geometry_of_a_Total_Solar_Eclipse.svg 52

Elizabeth Charlton, 2015

Solar eclipse types

!  Total solar eclipse !  Moon completely blocks Sun

!  Partial solar eclipse !  Moon blocks part of Sun

!  Annular solar eclipse !  Moon blocks center but not edge


Visual Examples

Lunar Eclipse

!  Definition !  Shadow of the earth falls on the moon

!  Total lunar eclipse – !  moon entirely in Earth’s shadow

!  Partial lunar eclipse – !  moon partially in Earth’s shadow


Lunar Eclipse Geometry

!  Where does the moon need to be compared to the earth and sun for a lunar eclipse to happen?

!  Moon is behind earth compared to sun !  Full moon phase


Lunar eclipse

"Geometry of a Lunar Eclipse" by Sagredo - Own work, images of Earth and Moon derived from NASA images. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Geometry_of_a_Lunar_Eclipse.svg#mediaviewer/File:Geometry_of_a_Lunar_Eclipse.svg


Why not every month?

•  The Moon’s orbit is tilted with respect to the ecliptic: –  Nodes – points where

the Moon’s orbit crosses the ecliptic

–  Eclipses only occur when the Moon is passing through a node


"Lunar eclipse diagram-en" by SuperManu - en:Image:Lunareclipsediagram2.gif by Tom Ruen, and derivating from Image:Solar eclipse fr.svg. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Lunar_eclipse_diagram-en.svg#mediaviewer/File:Lunar_eclipse_diagram-en.svg

Eclipse Calendar

•  Eclipses in the UK – http://www.timeanddate.com/eclipse/in/uk/

oxford – Next solar eclipse visible from the UK:

•  Partial Solar Eclipse •  20 March 2015

•  NASA Website !  http://eclipse.gsfc.nasa.gov/eclipse.html


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