The Sun: theory
- IPS / The Australian Space Weather Agency - The educational page provides a lot of answers on often asked question about the Sun, solar activity, spaceweather, geomagnetic disturbances, aurorae and solar eclipses.
- Marshall Space Flight Center - NASA's MSFC provides a very extensive, yet easy to comprehend description on the sun and its relation to Earth. Contains very good schematics and animations.
- David Craig - Presentation on the basics and vital statistics of the Sun.
- High Altitude Observatory - This page contains a lot of FAQ's as well as a page describing the history and other highlights of our understanding of the sun.
- WNCC - Handwalks you through some more difficult theoretical aspects like the solar spectrum, energy-production and solar rotation.
- Space Physics and Terrestrial Effects - This SEC-page deals with solar evolution, structure, observing in different wavelengths and interactions with the Earth. Most interesting are the detailed activities, including topics such as energy transport, building of a magnetometer or a spectroscope, and radiation hazards in space.
- Space Physics - Focuses on the sun's structure and the solar wind.
- Solar Views - This site provides essentials on the sun. It keeps a list of recent discoveries like solar quakes, the disappearance of the solar wind,...
- Wanderers in Space - This well-organized site provides lots of information on topics such as solar structure, magnetism, flares and the impact on Earth. Each time, the basics are covered in a list of facts, and more details and images are covered in the sections "tutorials" and "images". You'll find easy to understand coverage of concepts such as magnetic carpets, solar flare models, brightness of the corona,...
- The Sun - This UN-ESA cooperation discusses all solar aspects and includes some theoretical formulas.
- The Physics of the Sun - This presentation deals with all solar aspects, in particular neutrinos and solar flare models.
- Stanford Solar Center - In-depth coverage of a variety of solar topics (per level) like helioseismology, sundial, ancient solar observatories,... with lots of do-it-yourself activities and links to other websites.
- Yohkoh Public Outreach Project - Very interesting site of a not longer operational solar satellite that observed the Sun in X-rays from 1991 till 2001. Especially the "Science Nuggets" provide detailed information on the structure of the solar atmosphere.
- TRACE - This satellite observes the Sun in detail in the EUV. There are weekly pictures and movies that are supplemented by comprehensive comments. This is why rather theoretical concepts such as coronal loops, sigmoids and solar tornadoes soon reveal their secrets with these well-chosen images.
- RHESSI - Another solar satellite focusing on the science of solar flares by observing them in X- and Gamma-rays.
- Astronomy Notes - This site discusses astrophysical topics like neutrinos and the sun's power source without the use of formulas, but with the use of very clarifying animations.
- Oulu Space Physics Textbook - Among many other, topics that are discussed include plasma waves, heliosphere and cosmic rays.
- SOHO - The SOHO's website contains a news- and classroom with headlines, glossary, Dr. SOHO and extensive list of links.
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The time-distance between Sun and Earth
Because the speed of light is finite (299.792,458 km/s), it takes some time before the light of the sun reaches the earth. Moreover, as the spatial distance between the sun and the earth varies throughout the year, the time-distance will vary accordingly.
The graph underneath shows this variation for the year 2005. The right axis shows the distance between the center of the sun and that from the earth in million kilometers. The gravitational influences of the moon and the major planets were taken into account. The distance varies from 147,100 million km (02 Jan 05) to 152,102 million km (05 Jul 05).
The left axis shows the time necessary for the light of the solar photosphere to reach the earth's surface. This means that the above distance was corrected for the radii of the sun (696.000 km) and the earth (6.378 km). The time-distance varies in harmony with the earth's distance to the sun: it is lowest on 02 Jan 05 (488,33 seconds), and highest on 05 Jul 05 (505,02 seconds). On the average, the light from the sun needs 8 minutes 16,7 seconds to reach earth.
Some slight deviations in the time-distance may still be possible. The earth's globe is not exactly spherical, the sun is a plasma and hence has no well defined boundary. On top of this, convective movements in the photosphere due to oscillations, pulsations,... may change the radius of the sun. Nonetheless, it concerns only small deviations in the order of a few hundred kilometers maximum. Thus, their influence is negligible and the calculated time-distance is correct within at least one hundreth of a second.
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