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Observations and the Solar System : Lecture 2

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Stars and Wandering Stars

To an observer, in the night sky there are around 3000 bright points of lght. They appear to have no structure (unlike the sun and moon)
  • Like the sun they rotate about the night sky -this is due to the earths rotation. To an observer they appear to rotate about a fixed point - known as the North Pole . time-lapse photograph of the night sky shows this rather well and here Both Geocentric and Heliocentric cosmologies had the stars fixed at very large distances from the earth and not moving.)
  • Stars generally lie in fixed groups known as constelations. Constelations have ancient names.
  • Of the 3000 or so stars all except 5 stay in fixed patterns
  • These five are known as Wandering stars They were given names by the Greeks (or is it the Romans!)
    Days of the week in English,French and Welsh

    Motion of Planets

  • These wanderers lie close to the ecliptic.
  • These stars change position in the night sky in a non-simple way. Mars Jupiter and Saturn have Retrograde motion
    The retrograde motion of Mars in 2005 is shown here and Mars in 2003 here

    Interpretations

    From 2C AD Ptolomy proposed the Geocentric Cosmology which was a model of the universe (a cosmology) which had the earth at the centre with the moon, sun, and wandering stars orbitting the earth. Although this theory is wrong it actually fits the sun-moon-earth observations very well. Copernicus (1473-1543) proposed the alternate theory of the Heliocentric Cosmology which had the sun at the centre of the Universe. The Heliocentric view was that the planets and the earth were also rotating around the sun. Within the model
    1. Mars, Jupiter and Saturn orbit outside the earth's orbit
    2. Venus and Mercury Orbit within the Earth's orbit
    3. All planets rotate about the sun in the same direction
    4. The Earth is also spinning
    5. The time it takes for a full rotation - the period, which is 1 year for the earth increases as one gets further from the sun.
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    Heliocentric Cosmology

    Inner Planers: Venus, Mercury

    For Venus, for a planet within the earth's orbit we define
  • Superior Conjunction -when earth-sun-venus line up (in that order-Position A)
  • Inferior Conjunction -when earth-venus-sun line up (in that order-Position C)
    At both conjunctions Venus is not visable.(Hidden by the sun)
  • Greatest Eastern Elongation (position B) - when Venus is a far away (in angles) from the Sun as it gets. At this time Venus appears 47 degrees above the Horizon at sunset in the West.
  • Greatest Western Elongation (position D) - when Venus is a far away (in angles) from the Sun as it gets. At this time Venus appears 47 degrees above the Horizon at sunrise in the East. x

    Next Elongations for Venus

    Greatest Eastern Elongation Conjunction Greatest Western Elongation Conjunction
    June 6th 2015 Aug 15th 2015 Oct 26th 2015, Jun 6th 2016
    Jan 12th 2017 Mar 25th 2017 Jun 3rd 2017, Jan 9th 2018
    Aug 17th 2018 Oct 26th 2018 Jan 6th 2019, Aug 14th 2019
    Mar 24th 2020 May 13th 2020 Aug 13th 2020 Mar 26th 2021
    Oct 29th 2021

    Outer Planets: Mars Jupiter, Saturn

    We start with some more precise definitions with orbits. For the outer planets (mars, jupiter and saturn) we can define points on the orbit x Note that these descriptions assume that the planets orbits are circular and coplanar

    The dates of recent and (some) future opposions

    Mars Jupiter Saturn
    Oct 29th 2011 Apr 4th 2011
    Apr 8th 2014 Jan 5th 2014 May 10th 2014
    - Feb 5th 2015 May 23rd 2015
    May 22nd 2016 - Mar 8th 2016 June 3rd 2016
    - Apr 7th 2017 June 15th 2017
    27th July 2018 May 9th 2018 June 27th 2018
    - June 10th 2019 July 9th 2019
    Oct 13th 2020 July 14th 2020 July 20th 2020
    In the copernican model, the period of the planet increases with distsance from the sun: explaining retrograde motion ( see asnimation here) © Dave Dunbar 2020