1AU = 150,000,000 km

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1 ! 2.1i demonstrate an understanding of the terms: perihelion, aphelion, greatest elongation, conjunction, opposition, transit and occultation We know that the orbits of planets around the Sun are not circular, but are elliptical. This was stated by the astronomer Kepler in his first law. As a result, during one orbit, there must be a point where the Earth (or other planet) is closest to the Sun and another point where the planet is furthest away. The following diagram shows this :- PERIHELION CLOSEST TO THE SUN SUN PLANET APHELION FURTHEST AWAY FROM THE SUN If the orbit of the Earth was circular, the mean Earth to Sun distance would always be the same. This distance is called 1 Astronomical Unit (1AU) 1AU = 150,000,000 km Picture credits : (Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17

2 GREATEST ELONGATION This is the largest angular distance of Mercury or Venus from the Sun. This is only for the planets that orbit the Sun inside the orbit of the Earth. Greatest elongation is the best time to view these inner planets as they will be seen at their furthest angular distance from the Sun, well away from a line of sight with the Sun. Inferior planets (orbit between the Earth and the Sun) These will move across the sky, always being relatively close to the Sun. Mercury is only seen:- Before sunrise After sunset When the Sun is present in the sky, the glare from the Sun means the planet can no longer be seen. As Venus has a larger orbit, it can be seen further away from the Sun either:- Before or after dawn Before or after dusk VENUS MERCURY SUN (BELOW HORIZON)

3 Viewing Mercury Mercury can only be seen at a few times through the year, as it is often in a line of sight close with the Sun and impossible to see as a result. In the evening, about 30 to 45 minutes after sunset, or in the morning about 30 to 45 minutes before sunrise these are the only times you will be able to see Mercury when it is near to greatest elongation. Once the Sun is visible, the glare prevents Mercury being seen. The path that Mercury tracks across the sky from the horizon is similar to the path shown below (as for September/October 2016):- September 28 th GREATEST ELONGATION (morning) If you have never seen Mercury, the morning of 11th October 2016 is likely to be the ideal time to see the planet (weather permitting). You need a very low eastern horizon. Look out from 6am onwards. Jupiter will appear (easier to spot as it is very bright). Close by will be Mercury. September 21 st Horizon October 12 th Tip: Stand with your arm out straight and your thumb up. If you are trying to see Mercury, it will never rise above the tip of your thumb, with the base of the thumb level with a clear, low horizon. Viewing Venus Venus will usually be higher in the sky than Mercury because it has a bigger orbit. Venus can be seen as the bright evening star towards the west, or the bright morning star towards the east, being visible to magnitude This makes Venus one of the easiest objects to see as it can be extremely bright and stays in the sky over months at a time. Venus can be a considerable distance from the Sun in the sky and so the planet can appear in the sky even with the Sun visible.

4 Diagram showing the position of the inferior planets at greatest elongation Planet in the most favourable position to be observed (Mercury or Venus) View from Earth θ = 22 for Mercury θ = 47 for Venus θ is the angle for greatest elongation of either Mercury or Venus Picture credit : (Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17 The position of the planet in its orbit means that for each complete orbit there are 2 greatest elongations. One is as shown in the diagram and the other is when the planet is on the opposite side of the Sun. One gives the opportunity of seeing the planet after the Sun has set (greatest eastern elongation) and the other allows the planet to be seen before dawn (greatest western elongation). Dates for future greatest elongations (giving good viewing opportunities) are:- MERCURY September 28 th 2016 (am) April 1 st 2017 (pm) September 12 th 2017 (am) August 26 th 2018 (am) VENUS January 12 th 2017 (pm) January 6 th 2019 (am) March 24 th 2020 (pm) August 13 th 2020 (am)

5 Conjunction Conjunction is when 2 celestial objects are in line when viewed from Earth eg when a planet is in line with the Sun inferior planets could be in front or behind the Sun, whereas superior planets (planets orbiting the Sun outside Earth s orbit), can only be behind the Sun. The elongation angle of a planet at conjunction is 0. The planet cannot be seen because the Sun s bright light inhibits its view (except at a time of transit for the inferior planets, where special techniques are used for viewing). Earth Sun Outer Planet Superior Conjunction Inner Planet Superior Conjunction Inner Planet Inferior Conjunction Picture credit : (Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17 Superior conjunctions are when the planet is on the far side of the Sun from the Earth. Looking across the distance of the majority of an orbit, a planet will look small at times close to conjunction, when the planet is on the far side of the Sun. Inferior conjunctions can only occur with the inferior planets which have their orbit inside the orbit of the Earth, where Mercury or Venus pass on the near side of the Sun. When an inferior planet moves directly across the line of the Sun, a transit, the planet can be seen (but only using specialist equipment). AND NEVER WITH THE EYES LOOKING DIRECTLY AT THE SUN OR THROUGH AN OPTICAL INSTRUMENT

6 Opposition Opposition is when the Sun, the Earth and an outer planet are in line again, but we look away from the Sun at night to the outer planet. The planet can appear very bright because:-! The distance between the Earth and the outer planet is the smallest it can be, so the outer planet appears quite large! The planet is at its maximum brightness because the sunlight is reflected directly back to Earth :- Magnitudes: Mars -2.8 Jupiter -2.5 Saturn -0.3 Uranus 5.5 Opposition can only occur for the outer planets, as the inner planets will always be in the direction looking towards the Sun. Viewing of the outer planets is best at opposition. Earth Sun Picture credit : (Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17 Dates for future oppositions of the outer planets are:- MARS JUPITER SATURN URANUS NEPTUNE May 22 nd 2016 April 7 th 2017 June 15 th 2017 October 15 th 2016 July 27 th May 9 th June 27 th October 19 th October 13 th June 10 th July 9 th October 24 th December 8 th July 14 th July 20 th October 28 th years 7 weeks 1 year 1 month 1 year 2 weeks 1 year 4 days 1 year 2 days (Time between successive oppositions) Outer Planet Opposition September 2 nd 2016 September 5 th 2017 September 7 th 2018 September 10 th 2019

7 Transit The route of a smaller celestial body passing in front of a larger one eg Mercury or Venus in front of the Sun - or a moon in front of its planet eg Io in front of Jupiter. Picture credit : (Sun) SOHO/ESA&NASA A view of the transit of Venus on 8 th June 2004 Venus can clearly be seen passing in front of the Sun at about halfway through the transit. The photograph is of the image which had been projected on to a screen, using a pair of binoculars. NEXT MERCURY TRANSIT NEXT VENUS TRANSIT November 11 th 2019 (part will be visible in UK) December 11 th 2117 (not visible in UK)

8 Occultation The route of a celestial body passing behind another one eg when a planet blocks the view of its moon (Io passing behind Jupiter). Other examples are when a planet disappears behind the moon, a star disappears behind a planet, or an asteroid disappears behind the moon. Occultations are common events. Picture credit : (Moon) NASA-JPL The sequence of pictures below show the occultation of Jupiter behind the moon Picture credit : Becky Coretti and Bill Williams