Download 2.1d-f-g Planets in the zodiac, inclined to the ecliptic

2.1d
recall that the ecliptic is the plane of the Earth’s orbit around the Sun
2.1f
recall that planets move in elliptical orbits, slightly inclined to the ecliptic
2.1g
demonstrate an understanding that the planets appear to move within a
band called the Zodiac
What happens with no tilt to the Earth’s axis
CELESTIAL
EQUATOR
EQUATOR
This is the projection
of the equator up on
to the sky above
Picture credit : NASA Marshall Space Flight Centre (NASA-MSFC)
The CELESTIAL POLES would lie directly above the Earth’s poles:CELESTIAL NORTH POLE
The CELESTIAL POLES are the
projection of the Earth’s
poles up on to the sky above
CELESTIAL SOUTH POLE
Picture credit : (NASA-MSFC)
What happens with no tilt to the Earth’s axis
EQUATOR
THE CELESTIAL
EQUATOR AND THE
PATH OF THE SUN
WOULD APPEAR AS
A STRAIGHT LINE IN
THE NIGHT SKY,
WITH NO TILT TO
THE EARTH’S AXIS
Picture credit : (NASA-MSFC)
This would be the line along which the Sun would be seen to track across the stars
through the year
However the Earth’s axis is tilted by 23.5˚
What happens with a 23.5˚ tilt to the Earth’s axis
The Sun still moves along where the equator (NOW ANGLED AT 23.5˚) used to be
NORTH CELESTIAL POLE
CELESTIAL EQUATOR
23.5˚
SOUTH CELESTIAL POLE
THE DOTTED BLUE LINE SHOWS THE ORIGINAL LOCATION OF THE EARTH’S EQUATOR
WHEN PROJECTED ON TO THE NIGHT SKY
AS THE MOVEMENT OF THE SUN STILL APPEARS TO BE ABOVE THE ORIGINAL EQUATOR,
THE PATH OF THE SUN IS NOW ACROSS A DIFFERENT AREA OF THE CELESTIAL SKY IN
RELATION TO THE EARTH’S TILT
THIS IS THE ECLIPTIC
Picture credit : (NASA-MSFC)
The Earth’s axis which it spins on is at an angle of 23.5˚ compared to the plane of the
Earth’s orbit around the Sun. All the other planets (with the exception of Mercury
which moves away from the ecliptic plane by 7˚) move along almost the same plane as
they orbit the Sun in their elliptical orbits. Allowing for Mercury being either up to 7˚
above or below the ecliptic plane, all the planets move within a band of the sky (the
ZODIAC) which is a little more than 14˚ wide, to allow for the motion of the moon as well.
GEMINI
ARIES
VIRGO
PISCES
SAGITTARIUS
ECLIPTIC
ZODIAC
The angle of 23.5˚ creates a ‘wave' in the path of the Sun across the celestial sphere
The constellations that the Sun ‘passes through’ during a year are known as the
constellations of the zodiac. The ‘Signs of the Zodiac’ date from Babylonian times and
represent 12 of the constellations in the night sky found by the ecliptic (there is one
other constellation – Ophiuchus which cuts across this band). The diagram below
shows the sequence of the 13 constellations that the Sun passes in front of during a year.
CAPRICORN
AQUARIUS
SAGITTARIUS
OPHIUCHUS
SCORPIO
LIBRA
VIRGO
PISCES
LEO
ARIES
TAURUS
GEMINI
Picture credits : (Sun) SOHO/ESA&NASA, (Earth) NASA/JSC-Apollo17
CANCER
Some extra points of interest
The constellations of the zodiac appear at different times to the ‘Star Signs’ due to a
factor called PRECESSION – caused by the axis of the Earth having a slightly off-centre
motion, like the wobble of a spinning top toy. The precession occurs over about a
26,000 year cycle, meaning that the North Star changes during this time. In about
11,000 years time, the Pole Star will become Vega, as opposed to Polaris as at present.
(Synodic Periods)
When viewing a planet from Earth, the period between the times their positions both lie
on the same radial line from the Sun, is called the SYNODIC PERIOD. For planets closer
to the Sun than the Earth, the SYNODIC PERIOD is longer than the SIDEREAL PERIOD,
and for outer planets it is shorter.
The period which brings the Earth back to the same angular position with respect to
the Sun is called the tropical year and is 365.242 mean solar days. Formally this period
is defined as the interval of time from one SPRING/VERNAL EQUINOX to the next. The
sidereal period (the period with respect to the distant stars) of 365.256 mean solar
days is about 20 minutes longer because of the precession of the Earth's spin axis.
The precession period of about 26,000 years brings the vernal equinox about 20
minutes earlier each year. Star catalogues are written with new Right Ascension values
every so often to allow for this. An atlas of the night sky is produced with a statement
‘Epoch 2,000’, meaning that the RA values are for the start of the millennium. Previous
atlases have shown ‘Epoch 1950’ to show the values nearer to that year.
The fact that the year is not exactly 365 days has led to the inclusion of leap year days
and other adjustments to the calendar. At midnight on 4th October 1582, errors from
the calendar made in Julius Caesar’s time (the Julian calendar) were corrected by Pope
Gregory (the Gregorian calendar) by missing 10 days - 4th October was followed by
15th October!
Many countries
did not
immediately follow this rule and so for many
years there was a great deal of confusion. These days time is judged against an atomic
clock and leap seconds are used for the ever so slowly lengthening Earth day – the rate
of spin of the Earth is slowing down due to the effect of the tides.