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Sun and Planetary Motion Revision Notes

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22.2Sun and Planetary Motion


In these revision notes for Sun and Planetary Motion, we cover the following key points:

  • What are some basic features of the Sun (mass, radius, position)?
  • Why is Sun the main source of energy for Solar System?
  • How the solar energy is produced?
  • How energy transmission does occur in different layers of the Sun?
  • What is the structure of Sun? What features does each layer present?
  • What are solar protuberances?
  • What laws the planetary motion around the Sun does follow?
  • What is sidereal and tropical period?
  • What is ecliptics and ecliptic plane?
  • What are inner and outer planet? How does the observation angle varies with position?
  • How do planets rotate around their own axis?
  • What are seasons/solstices/equinoxes?
  • How day and night are formed? How do they vary in different seasons and locations?

Sun and Planetary Motion Revision Notes

The Sun is a fiery giant orb of mass M = 1.9989 × 1030 kg, mostly made up of hydrogen and helium. The Sun is an average sized star formed about 4.6 billion years ago. The radius of the Sun is 6.96 × 105 km. The Sun is located at centre of our Solar System and it is the main source of energy for it. All planets are "fed" by portions of solar energy incident from the Sun.

The composition of the Sun consists of 71% hydrogen and 27% helium. The rest of 2% are other chemical elements.

The proton-proton reactions necessary to produce solar energy occur in the innermost part of the Sun called the "Sun core". It is a spherical-shaped region of radius not more than 1/5 of the Sun's radius.

The gamma photons produced in the Sun's core shift towards the surface of the Sun, where they radiate in all directions. Another method of solar energy transmission is through convection, i.e. through gas currents.

The Sun structure consists on the following layers: core (at centre), radiative zone (including the core), convection zone, photosphere, chromosphere and corona.

Sunspots represent an interesting phenomenon occurring in photosphere. They are darker regions than the other parts of the Sun due to the lower temperature (1000 K - 1500 K colder than the neighboring regions). Their presence is explained through magnetic fields present in the Sun. The number of such spots varies periodically every 11.2 years. When the number of sunspots is at maximum, the Sun is at peak of its magnetic activity.

Solar protuberances are flame bolts flowing out from chromosphere and turning back into chromosphere again. This occurs due to the effect caused by the Sun's magnetic field.

The Sun revolves around its own axis. The presence of sunspots makes the study of this phenomenon easier. The solar self-rotation period is 24.47 days at the equator and almost 38 days at the poles. The average rotation is 28 days.

All planets of our Solar System adhere to Kepler Laws of planetary motion.

The angle of the Earths revolution around the Sun depends on the frame of reference. If stars are taken as reference frame, the corresponding period of revolution is known as sidereal (stellar) period, while when the Sun is taken as a reference frame, then corresponding period is known as tropical (solar) period. These periods (that determine the length of the corresponding years) are not the same; they have small deflections due to the effect of the Moon on the Earths revolution around the Sun. Thus, the duration of one sidereal year is 365.2568 days and that of a tropical year is 365.2422 years.

Since all planetary orbits are nearly at the same plane when observed from Earth, planets are aligned with a unique arc in space. The plane containing this arc is known as ecliptic plane and it divides the sky in two parts. More specifically, the ecliptic plane is defined as the imaginary plane containing the Earth's orbit around the sun. If we consider the imaginary line that connects the centre of Sun, Earth and all the other planets of they are all aligned, it is called ecliptics.

Since Mercury and Venus are the only planets inside the Earth's orbit (they are closer to the Sun and their orbit is smaller than Earth's orbit), they are called inner planets. All the other planets, from Mars to Neptune are called outer planets since their orbits include that of Earth.

In addition to orbital motion (revolution) around the Sun, planets rotate around themselves as well. This rotation occurs according an axis passing through the centre of planet, known as planetary axis. The plane of Earth planetary axis is deflected at 23.45° to the ecliptics plane.

When stars are taken as a (stationary) reference frame, the Earth completes one rotation around its own axis in 23 h 56 min. A solar day is slightly longer; it is about 24 hours.

The phenomenon of seasons occurs because of the angle formed by the rotation axis of the Earth and the orbital plane. This inclination makes solar radiation fall at different angles on a specific point of Earth at various period of the year. Since light intensity depends on the incidence angle, the solar energy brought to Earth is not the same throughout the year.

Solstice represents the time when the angle of incidence of sunrays on a region of Earth is the lowest or greatest possible. The day in which this angle is the lowest, is the longest day of the year in that place, and the summer begins.

An Equinox occurs when the Earths axis is only deflected laterally (not forwards or rearwards) in respect to the orbital plane. We have two equinoxes: in the northern hemisphere there is spring equinox on March 21st and autumn equinox on September 23rd. On these days, the duration of day and night are equal.

Day represents the time in which a region of the Earth is directly exposed to sunrays. Night is the opposite, i.e. a region is either illuminated indirectly from the Sun (through Moon surface that reflects the sunrays on to the Earth) or by means of other light sources (usually artificial, such as electric bulbs).

All locations on the equator have always have an equal duration of day and night as sunlight always falls on them at the same angle.

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