Unified and separate forces
A close conjunction between
Saturn and Venus.
Adapted from an article in (British) Astronomy Now magazine by Iain Nicolson
Five planets have been known since antiquity...


Five of the planets in our solar system are sufficiently bright to be visible to the naked eye. These planets are: Mercury , Venus, Mars, Jupiter and Saturn. Although their brightness varies as their positions relative to the Earth change, at their most brilliant, all of them, except for Saturn, become brighter in our sky than the brightest star (Sirius). Even Saturn, most distant of the naked eye planets, can attain an apparent magnitude as bright as -0.3, comparable to the third brightest star (alpha Centauri) and inferior only to Sirius and Canopus.


Mercury and Venus are known as the inferior planets because they lie closer to the Sun than the Earth is. Moving along their orbits faster than the Earth, each overtakes the Earth at regular intervals.
At greatest elongation, the angle between the Sun and Venus is about 47 degrees and Venus can rise or set more than three hours before or after the Sun. But Mercury, which lies significantly closer to the Sun than Venus, remains at all times rather close to the Sun in the sky and usually can be seen with the naked eye, under good conditions only for a few days around each greatest elongation.
Because Mercury's orbit is more markedly elliptical than that of any of the other naked eye planets, its greatest elongation can range from as little as 17 degrees to a maximum of 28 degrees. The smallest value of greatest elongation occurs near perihelion (the point in its orbit at which Mercury is closest to the Sun) and the largest value of greatest elongation occurs when Mercury is close to aphelion (maximum distance from the Sun).

Orbit Plane diagram
The orbit of a planet crosses the plane of the Earth's orbit at
two points: the ascending and descending nodes. A transit will
be seen only if inferior conjunction occurs when the planet is
at (or close to) one of the nodes, such as E1, P1 or E3, P3.
Synodic period diagram
(a) The motion of an inferior planet (P) is shown relative to the Sun (S) and the Earth (E): P1 = inferior conjunction; P2 = greatest elongation west: P3 = superior conjunction: P4 = greatest elongation east.
(b) When the planet is to the west of the Sun (as at P2), it rises before the Sun.
(c) When the planet is to the east of the Sun (as at P4), it sets after the Sun.


If the orbital planes of the inferior planets coincided exactly with the plane of the Earth's orbit the planets would pass directly between the Earth and the Sun and would be visible as small dark discs crossing the face of the Sun. But, because the orbit of Mercury is tilted (inclined) at an angle of seven degrees, and that of Venus is tilted 3.4 degrees, each passes a little way above or below the Sun at inferior conjunction. Only if inferior conjunction occurs when a planet is at, or sufficiently close to, one of the nodes of its orbit (the two points at which the planet's orbit crosses the plane of the Earth's orbit) will the planet be seen across the face of the Sun. Such an event is called a transit.
Transits of Venus are rare events. The last two occurred in 1974 and 1882 and the next will not occur until 2004 and then again in 2012. Transits of Mercury are more common. The last occurred on November 6, 1993, and the next is due on November 15, 1999. Unfortunately, because mid-transit occurs at 21:42 UT (at night) the event will not be visible from Western Europe.


Mars, Jupiter and Saturn lie further from the Sun than Earth does. These planets move more slowly along their orbits than Earth. Therefore, the Earth periodically catches up with and overtakes, each of them. On such occasions, the planet that's being overtaken by the Earth, lies on the opposite side of the Earth from the Sun. This position therefore is called opposition.
Around the time of opposition, a planet is visible for all or most of the night, rising around the time of sunset and setting around the time of sunrise (the precise rising and setting times depend on the tilt of the planet's orbit, the season of the year and the latitude of the observer), and is at its closest to the Earth.
Thereafter, as the Earth moves ahead of the planet, the planet appears to drift closer to the Sun in the sky, setting in the west progressively earlier until it is lost in the glow of the setting Sun. Viewed from the Earth, the planet then passes behind the Sun (it is then at superior conjunction) before beginning to emerge on its western side and becoming visible in the eastern sky before sunrise.
As the Earth begins to catch up with the planet again, the angle between the Sun and planet (the elongation) increases, and the planet rises progressively earlier than the Sun. When the elongation becomes 180 degrees, the planet is once again at opposition.
Mars was last at opposition on April 24, 1999, and will next arrive at opposition on June 13, 2001. Jupiter reached opposition on October 23, 1999 and Saturn on November 6, 1999.


Synodic period diagram
Synodic period:
Opposition occurs when the
planet is at P1 and the Earth
at E1.
After one year, the Earth has
made one complete circuit of
the Sun and arrived at E2,
by which time the planet has
moved to P2.
Opposition will not recur until
the Earth has caught up with
the planet, which it does
when the planet is at P3
and the Earth at E3.

Opposition diagram
Superior planet:
when the planet is at position P1,
it is at opposition.
When it is at P2, it is at
superior conjunction.

When an inferior planet passes between the Earth and the Sun, the planet is at inferior conjunction (a conjunction is a close alignment of two bodies as viewed from the Earth). Thereafter it moves to the west of the Sun in the sky, causing the angle between the Sun and the planet (the elongation) to increase toward a maximum value (greatest elongation west), which occurs when our line of sight looks along a tangent to the inferior planet's orbit. After that, the elongation then decreases until the planet passes behind the Sun, at which point it is said to be at superior conjunction.
Thereafter, the inferior planet begins to emerge on the eastern side of the Sun, moving out towards greatest elongation east before again beginning to approach the Sun as it advances towards the next inferior conjunction.
While the planet is to the west of the Sun, it rises before the Sun and so it is visible to the naked eye in the eastern sky before dawn. When it is on the eastern side of the Sun it sets after the Sun, and so is visible in the evening sky after sunset.
An inferior planet is best placed for observation when it is close to greatest elongation because the angle between the planet and the Sun is a maximum and, therefore, the difference between the rising and setting times of the Sun and the planet are also maximum.
The mean time interval between two successive similar alignments of a planet - for example, between two oppositions or two inferior conjunctions, or two similar elongations, is called the synodic period. The time taken by a planet to complete one orbit around the Sun, by contrast is called the sidereal period.
The synodic period depends on the relative angular motion of the planet and Earth. For example, the Earth, which has a sidereal period of 365.25 days, moves around the Sun at an average angular velocity of 0.9856 degrees per day (360 degrees / 365.25 days). Mars, with a sidereal period of 686.98 days travels more slowly, at an average angular rate of 0.524 degrees per day.
The relative angular motion of the Earth and Mars is the difference between their angular rates, i.e. 0.9856 - 0.524 = 0.462 degrees per day.
The average time interval between two successive oppositions of Mars (in other words, the synodic period of Mars) is the time that it takes for the angle between Earth and Mars to change by 360 degrees and is therefore equal to 360 / 0.462 = 780 days.
Because Jupiter and Saturn move much more slowly than does the Earth, their synodic periods are considerably shorter than that of Mars: 399 days and 378 days, respectively. Consequently, Jupiter returns to opposition about one month later and Saturn about two weeks later, on each successive year. Mars, by contrast, returns to opposition at average intervals of about 2.14 years.
The synodic periods of Mercury and Venus are 116 and 584 days respectively. So, on average, Mercury returns to greatest elongation east (evening visibility) at intervals of just under four months, whereas Venus will take about 1.6 years.


During the last few months of 1999, the naked eye planets were well placed for observation, with the exception of Mars, which was low in the evening twilight.

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Copyright 1999-2000; Luis M. Luis. All Rights Reserved.