Download conjunction and opposition

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
PLANET
SUN
!
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
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)
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.
Horizon
September
21st
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 - 4.5. 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.
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
VENUS
September 28th 2016 (am)
January 12th 2017 (pm)
April 1st 2017 (pm)
January 6th 2019 (am)
September 12th 2017 (am)
March 24th 2020 (pm)
August 26th 2018 (am)
August 13th 2020 (am)
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
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
Outer Planet
Opposition
Picture credit : (Sun) SOHO/ESA&NASA (Earth) NASA/JSC-Apollo17
Dates for future oppositions of the outer planets are:MARS
JUPITER
SATURN
URANUS
May 22nd
April 7th
June 15th
October 15th
2016
2017
2017
2016
July 27th
May 9th
June 27th
October 19th
2018 https://in-the-sky.org/newsindex.php?feed=13&day=8&month=12&year=2022
2018
2018
2017
th
th
th
October 13
June 10
July 9
October 24th
2020
2019
2019
2018
th
th
th
December 8
July 14
July 20
October 28th
2022
2020
2020
2019
2 years 7 weeks
1 year 1 month
1 year 2 weeks
1 year 4 days
(Time between successive oppositions)
NEPTUNE
September 2nd
2016
September 5th
2017
September 7th
2018
September 10th
2019
1 year 2 days
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 8th 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 11th 2019
(part will be visible in UK)
December 11th 2117
(not visible in UK)
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