Astronomy Answers: Planetary Phenomena

Astronomy Answers
Planetary Phenomena


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1. Perigee ("p" in the tables) ... 2. Apogee ("a" in the tables) ... 3. Greatest Northerly Declination ("+d" in the tables) ... 4. Greatest Southerly Declination ("-d" in the tables) ... 5. Conjunction With The Sun ("c" in the tables) ... 6. Opposition to the Sun ("o" in the tables) ... 7. Greatest Elongation ("e" in the tables) ... 8. Passage Through a Node ("xa" and "xd" in the tables) ... 9. Smallest Conjunction Spread ("b" and "B" in the tables) ... 10. Greatest Conjunction Spread ("s" and "S" in the tables) ... 11. Prograde and Retrograde Motion ("Pg" and "Rg" in de tables) ... 12. Acceleration ("V" and "v" in the tables)

A number of major phenomena of the planets Mercury - Neptune that are visible from Earth at some point during the years 1885 through 2105 are listed in diagrams and tables on one page for each year, which you can reach through the list given below. The meaning of the phenomena is explained lower down. The diagrams are explained on the page about the year 2000.

Years:

1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045 2046 2047 2048 2049 2050 2051 2052 2053 2054 2055 2056 2057 2058 2059 2060 2061 2062 2063 2064 2065 2066 2067 2068 2069 2070 2071 2072 2073 2074 2075 2076 2077 2078 2079 2080 2081 2082 2083 2084 2085 2086 2087 2088 2089 2090 2091 2092 2093 2094 2095 2096 2097 2098 2099 2100 2101 2102 2103 2104 2105

1. Perigee ("p" in the tables)

A planet is in its perigee when the planet is closest to the Earth. That means that the planet was further away just before and just after that moment. When a planet is in its perigee then that planet appears largest in the sky (seen through a telescope).

A perigee is not favorable for watching an inferior planet, because such a planet has its perigee always close to a conjunction (namely, the inferior conjunction).

A perigee is favorable for watching a superior planet, because the perigee of such a planet is always close to the opposition of that planet, and then the planet also appears brightest in the sky.

2. Apogee ("a" in the tables)

A planet is in its apogee when that planet is furthest from the Earth. That means that the planet was closer just before and just after that moment. When a planet is in its apogee then that planet appears smallest in the sky and the least bright. Moreover, an apogee is never far from a conjunction (for inferior planets more specifically the superior conjunction), so it is all in all not a good time for watching the planet.

3. Greatest Northerly Declination ("+d" in the tables)

A planet has reached its (locally) greatest northern declination if the declination was less northerly just before and just after that moment. A planet can have a greatest northerly declination which is yet south of the celestial equator, just as long as the declination was even less northerly just before and just after that moment.

A large northerly declination of a planet is favorable for watching the planet from places north of the tropics. For observers there, the planet gets higher in the sky and remains longer above the horizon the more northerly the declination of the planet is.

For observers near the South Pole, such a planet remains below the horizon. For other observers south of the tropics, a large northerly declination is unfavorable, because the planet then stays lower in the sky and remains above the horizon for a shorter period of time each day.

4. Greatest Southerly Declination ("-d" in the tables)

A planet has reached its (locally) greatest southerly declination if the declination was less southerly just before and just after that moment. A planet can have a greatest southerly declination that is yet north of the celestial equator, just as long as the declination is less southerly just before and just after that moment.

A large southerly declination is favorable for watching south of the tropics. For observers there, the planet gets higher in the sky and remains longer above the horizon the more southerly the declination of the planet is.

For observers close to the North Pole, such a planet always remains below the horizon. For other observers north of the tropics, a large southerly declination is unfavorable, because the planet then remains lower in the sky and stays above the horizon for a shorter period of time each day.

5. Conjunction With The Sun ("c" in the tables)

A planet is in conjunction with the Sun if the difference between the geocentric ecliptic longitude of the planet and of the Sun is smallest, so it was greater just before and just after that moment. Inferior planets have two separate conjunctions each synodical period: an inferior conjunction (always close to perigee) when they pass between the Sun and the Earth, and a superior conjunction (always close to apogee) when they pass behind the Sun. Superior planets have only a single conjunction per synodical period, when they pass behind the Sun.

A conjunction is always unfavorable for watching the planet, because the planet is then very close to the Sun in the sky, and hence not visible at night. Inferior conjunctions of inferior planets are interesting in exceptional cases, namely when such a planet moves across the disk of the Sun, as seen from Earth. Something like that happens only if such a planet then also goes through a node of its orbit. This happened to Mercury on 14 November 2002 and will happen to Venus on 8 June 2004 and 6 June 2012.

6. Opposition to the Sun ("o" in the tables)

A superior planet is in opposition to the Sun if the difference between the geocentric ecliptic longitude of the planet and of the Sun is closest to 180 degrees, so it is less just before and just after that moment. Inferior planets are never in opposition.

An opposition is a favorable time to watch a planet, because then the planet is directly opposite the Sun in the sky, so it is above the horizon all night long. An opposition is also always close to a perigee, when the planet appears largest and brightest in the sky.

7. Greatest Elongation ("e" in the tables)

An inferior planet has reached its greatest elongation if the difference between the geocentric ecliptic longitude of the planet and that of the Sun is less just before and just after that moment.

The elongation is roughly equal to the distance, in the sky, of the planet from the Sun, measured in degrees. The greatest possible elongation of a superior planet is always 180 degrees, when the planet is in opposition and therefore as far as possible from the Sun in the sky. Because the greatest elongation of a superior planet is always the same, it is usually not considered to be interesting.

An inferior planet is never in opposition and can therefore not reach an elongation of 180 degrees. Such a planet moves away from the Sun (in the sky) after a conjunction, reaches a certain greatest elongation somewhere between 0 and 180 degrees, and then returns to the Sun. After another conjunction, the planet moves away from the Sun on the other side, reaches another greatest elongation, and then returns to the Sun once more, after which the whole story repeats itself. The greatest elongation of an inferior planet is never the same twice in a row, and is therefore interesting.

The time around the greatest elongation is a favorable time to watch a planet, because then that planet is furthest from the Sun and hence visible the longest before sunrise (if the elongation from the table is negative) as a morning star, or the longest after sunset (if the elongation is positive) as an evening star. Usually, an inferior planet is also the brightest around the time of its greatest elongation. Closer to the superior conjunction the planet is less bright because it is then further away from Earth, and closer to the inferior conjunction the planet is less bright because we then look mostly at its dark side.

8. Passage Through a Node ("xa" and "xd" in the tables)

The orbits of the planets are not in the same plane (the ecliptic) as the orbit of the Earth, but they do all have the Sun in the plane of their orbit. This means that each planet other than the Earth is sometimes above the plane of the Earth's orbit and sometimes below it, and that their orbit crosses the plane of the Earth's orbit. The intersections of an orbit with the ecliptic are called the nodes of the orbit. Well-behaved orbits have two nodes. If the planet goes through the ascending node of its orbit (indicated by "xa"), then it ends up on the north side of the ecliptic, and if it goes through the descending node of its orbit (indicated by "xd"), then it ends up on the south side of the ecliptic.

The nodes of an orbit are important for eclipses of or by the Sun or the planet. If an inferior planet passes through a node of its orbit just when it also has an inferior conjunction with the Sun, then the planet will pass across the solar disk as seen from Earth. This is a rare occurrence, which interests even scientists. Because we then look at the dark side of the planet while it travels across the bright disk of the Sun, the planet can then be observed from Earth (using special equipment). This happened to Mercury on 14 November 2002, and to Venus on 8 June 2004 and 6 June 2012.

If an interior planet goes through a node just when it also has a superior conjunction, or if a superior planet goes through a node just when it also has a conjunction with the Sun, then that planet will be eclipsed by the Sun. In practice, such a planet is invisible from Earth even just before or after such an eclipse.

9. Smallest Conjunction Spread ("b" and "B" in the tables)

The conjunction spread is a measure for how close together in the sky the planets are. The theory behind it is explained on the page about conjunctions. The conjunction spreads that are used in the tables are those of Mercury through Saturn ("b"), which can be viewed without a telescope or binoculars, and of Mercury through Neptune ("B").

The planets reach a smallest conjunction spread if the conjunction spread was greater just before and after that moment. When the conjunction spread is small, then the planets are close together in the sky. That can look pretty but has no other influence on Earth. When the planets are all close together, then they are above the horizon all at about the same time, and below the horizon (i.e., invisible) all at about the same time. There is then a reasonable chance that at any given moment you can see either many planets or no planets at all in the sky.

10. Greatest Conjunction Spread ("s" and "S" in the tables)

The planets reach a greatest conjunction spread if the conjunction spread is smaller just before and just after that moment. When the conjunction spread is large, the planets are far apart in the sky. That means that there is a good chance that at least one planet is above the horizon at any given moment. In the tables, "s" refers to the conjunction spread of Mercury - Saturn, and "S" to Mercury - Neptune.

11. Prograde and Retrograde Motion ("Pg" and "Rg" in de tables)

The planets usually move to the East relative to the stars in the sky, but sometimes (around their date of opposition to the Sun) to the West. Motion in the normal direction is called prograde, and motion in the opposite direction retrograde. In the tables, "Pg" means that the motion becomes prograde, and "Rg" that the motion becomes retrograde.

12. Acceleration ("V" and "v" in the tables)

A "v" in the table means that the acceleration of the planet changes from eastward (prograde) to westward (retrograde). A "V" means that the acceleration changes from westward to eastward.



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Last updated: 2016-02-07