Week 3 The Solar System
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Some of the topics in this session are devoted to our sun and its planets that make up the solar system. I have prepared another U3A course on this topic to which you can refer through the Astronomy Awareness page here. These contain a scale model of our system, based on the city of Canberra.
The sun
The sun is our nearest star, but until recently we understood more about distant suns than our own. Several web sites provide up-to-date information on our nearest star and we will use these to complete this section of Week 3.
The following link provides details of the NASA/European Space Agency's Solar and Heliospheric Observatory (SOHO) satellite, which is orbiting the sun inside the earth's orbit to collect data from the sun with a wide range of instruments. Our knowledge of the sun has been increased dramatically from the data obtained. Press releases and images of the sun are available from this site.
The
planets
Here is an important web link to the nine planets starting with an introduction by Bill Arnett to the fascinating facts about the solar system. Bill's introduction provides the links to which I refer later, and you will find that Bill's data is probably more recent than that used in my Astronomy Awareness course.
Finding
planets in the night
sky
We need to know which planets are visible in our night sky because each are wanderers, the name used by the ancient astronomers when they first observed that certain "stars" moved their position from month to month relative to the general field and pattern of stars. The easiest way to locate a star is to use a modern ephemeris or a publication that gives the night sky for each month at your location. You will remember that in Week 1, we discussed several methods to find your longitude and latitude. These can now be used to define which planets are visible and where to look.
Most astronomical societies or associations provide a Handbook or Ephemeris for each year, or you can buy a CD-ROM that gives the position of each planet for each night at your exact location. I use the Maris REDSHIFT CD or software such as TheSky Version 5.
If you do not have access to any of these, a useful publication is the yearly Practical Guide to the Night Sky by Quasar Publishing, and of course, you can use the US Naval Observatory Online ephemeris discussed in Week 1.
Observation hazards
Warning! Never use an instrument to view the sun directly because doing so will result in immediate eye damage and possible blindness. If you wish to see a solar eclipse, sun spots or a transit of a planet across the sun's disk, always project the image of the sun onto a shaded screen placed beyond the telescope eyepiece. Heat will be generated and you must focus carefully to prevent overheating of the eyepiece. If you want to follow this up please e-mail me for more information
Specific
Observing of the Planets
Mercury : Observation of the planet Mercury is best attempted in the early evening just after sunset or early morning just before sunrise. Mercury exhibits a phase as it is between the earth and sun and it will appear in the telescope (or binoculars) as a very small crescent. Remember to keep clear of the sun to avoid eye damage, so if this is an early attempt, always look for Mercury when the sun is set.
Venus : Observing Venus is much easier than observing Mercury, again wait until the sun is below the horizon and seek this planet in the west after sunset. It will normally be a very bright object, appearing as a crescent but the telescopic image will be fuzzy and not well focused due to atmospheric seeing.
Mars : Mars moves about the sky and can be very high on the ecliptic sometimes almost overhead. When in opposition with the earth (when the two planetary orbits allow each planet to be closest to each other) it will appear as a reddish disk. If you use 250 magnification, you may distinguish quite small, but at favourable oppositions dark, markings on the surface. It is not possible to see the two moons or satellites of Mars as they are far too small for our telescopes.
Jupiter : Probably the best object for beginners (other than the moon) with the planet generally high in the sky and second in brightness to Venus. You should see the atmospheric bands (or zones) of the planetary disk, and on rare clear nights the dark red spot near the equatorial regions of this, the largest planet of the solar system. The four brightest satellites of Jupiter can always be seen and as they form a miniature system of their own the satellites can be seen to move in position from week to week. Sometimes the shadow of these satellites can be seen on the planet's surface. Transits of the larger satellites can also be seen and occultations of each occur in a very regular pattern. Jupiter is always seen as a disk and not a crescent.
Saturn : This planet has a magnificent ring system which can be seen in any telescope. Its size including the bright rings is larger than Jupiter so it forms a very interesting object that may particularly interest children. The satellites of Saturn are much more difficult to observe, but the largest can be seen at around 300 magnification on clear nights.
Uranus : Not easy to find and observe due to its great distance from the sun, but once found, Uranus appears as a bright star. Most small telescopes can not resolve Uranus as a disk, but a energetic attempt can be made to find it once you know where it is in the sky.
Neptune : Similarly, you may locate Neptune as an exercise, but you will not see it as well as you can in modern Internet websites.
Pluto : This planet is for professional telescopes and is best studied by the Internet links, particularly the Hubble telescope animation.
These are the true wanders of the solar system and can appear at any time. Most are too faint for small telescopes but every now and again bright comets appear, some even too large to grasp by a small telescope. In South Australia I once saw an early morning comet that stretched almost from horizon to horizon, a most magnificent sight. Here is an overview link on these "visitors" from the most distant parts of the solar system:
Satellites are the companions of our solar system planets. You would have read about some of these in my Astronomy Awareness course, but to observe them, we generally require large telescopes. However, the Galilean satellites of Jupiter are bright enough for us to observe their occultations, transits and shadow transits across Jupiter's cloudy face. Here is a descriptive link that provides the details of Jupiter's satellites, and explains the terms used.
Very recent research has focused on new techniques to discover planetary bodies other than our solar system planets. Little information can be found by these techniques on the planetary possibility of life, but generally their size, orbital details and mass can be determined. Here is a link that provides the latest information on this fascinating astronomical research.
I don't doubt that you will need to re-read this section several times before proceeding to the next topic. Indeed, I had to refresh the ideas myself when I started giving astronomy lessons in U3A.
Questions for
reflection
Look back through these notes or use other reference material, including Sobel's book and the websites and see if you can answer the following questions. If you have any problems e-mail me or put your query on the discussion page.
What is our sun? How does it support life on earth?
What does the term transit mean? What is the difference between the transit of Mercury and that of the satellite Io?
What is the importance of gravity in considering the satellites of Jupiter?
How can you find Jupiter's satellites in the night sky?
In what ways can observing be harmful to astronomers?
How would you find information on Pluto? Is this the most distant object in our solar system?
Activity
http://aa.usno.navy.mil/AA/data/
This US Naval Observatory website is a comprehensive indicator of astronomical events. You can use this website to find the mutual occultations of Jupiter's bright satellites. You will need to save or print the results before using it later to observe these satellites from home. Some input requirements may be difficult, such as your time zone (but use your longitude in degrees divided by 15 to get the zone in hours) so don't worry if you can't work them out. I can provide the answer and give other help if you e-mail me your location, or home town.
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