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A sol is the apparent interval between two successive returns of the Sun to the same meridian (sundial time) as seen by an observer on Mars. It is one of several units for timekeeping on Mars. A sol is slightly longer than an Earth day. It is approximately 24 hours, 39 minutes, 35 seconds long.
The primary, secondary, tertiary, and quaternary divisions of the sol are based on the number 12. Sols are numbered 0 through the end of the year, with no additional structure to the calendar. The epoch is "the union of all races and nations in a single State, a union which was formally established 13,218 years ago".
The basic time periods from which the calendar is constructed are the Martian solar day (sometimes called a sol) and the Martian vernal equinox year.The sol is 39 minutes 35.244 seconds longer than the Terrestrial solar day, and the Martian vernal equinox year is 668.5907 sols in length (which corresponds to 686.9711 days on Earth).
The first satellites designed for long term observation of the Sun from interplanetary space were NASA's Pioneers 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field.
The Sun is part of one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur. [270] [271] It is a member of the thin disk population of stars orbiting close to the galactic plane. [272] Its speed around the center of the Milky Way is about 220 km/s, so that it completes one revolution every 240 million years. [269]
Predicted to be a region of hot hydrogen, a structure called the "hydrogen wall" may be between the bow shock and the heliopause. [50] The wall is composed of interstellar material interacting with the edge of the heliosphere. One paper released in 2013 studied the concept of a bow wave and hydrogen wall. [51]
The other type of commonly used "rotation period" is the object's synodic rotation period (or solar day), which may differ, by a fraction of a rotation or more than one rotation, to accommodate the portion of the object's orbital period around a star or another body during one day.
[a] [9] [12] One of these collapsing fragments (known as the presolar nebula) formed what became the Solar System. [13] The composition of this region with a mass just over that of the Sun ( M ☉ ) was about the same as that of the Sun today, with hydrogen , along with helium and trace amounts of lithium produced by Big Bang nucleosynthesis ...