What’s in the Sky in September
September
- 1st Saturn is just past Opposition ( closest psotion to Earth, which was on 27th August and is a stunning evening object at Magnitude o.4, distance 1309.9 milliom km, 72.82 light minutes away.
- 13th Moon at Apogee, furthest point from Earth this month, 01:43hrs AEST, distance from Earth 402,292.4 km.
- 23rd Southward Equinox 16:50hrs AEST.
- 28th Moon at Perigee, Closest point to Earth this month, 11:01hrs AEST, distance from Earth 359,914.3km.
- Universal Nomenclature of the Equinoxes and Solstices Each celestial object has coordinates similar to a street directory to locate its’ position in the heavens.Celestial Sphere – An imaginary sphere surrounding the Earth on which the celestial bodies appear to lie.Celestial Longitude (CLong) – coordinateon the Celestial Sphere that corresponds tothe longitude on Earth and is measured
from 00.00h. Measured in hours, minutes, seconds or sometimes given as degrees,
minutes seconds corresponding to degrees
in a circle, ie 360 degrees.
Celestial Latitude (CLat)– corresponds to latitude on Earth’s surface and is measured
in degrees, minutes, seconds. Up to 90
degrees North (Nth) or South (Sth) of the Celestial Equator.
+ for the Northern Hemisphere,
– for the Southern Hemisphere.
Celestial Equator – directly above the
Equator on the Earth’s surface.
Northward Equinox – Day & Time the
Sun crosses the Celestial Equator travelling into the northern Hemisphere. 00.00h CLong
North Solstice – Date and Time Sun reaches its’ furthest northern position ~ 23.5 degrees Nth. of the Celestial Equator. The Tropic of
Cancer on the Earth’s surface, northern limit
of Sun overhead.
Southward Equinox – Day and Time
Sun crosses the Celestial Equator travelling into the Southern Hemisphere.
South Solstice – Date and Time the Sun
reaches its furthest s outhern position ~ 23.5 degrees Sth. of the Celestial Equator.
The Tropic of Capricorn on the Earth’s
Surface, southern limit of Sun overhead.
The preceeding 4 terms indicate:
1.Direction of the Earth’s Poles
relative to the Sun.
2.Extent of the Nth/Sth movement
of the Sun overhead
3.Direction of seasonal change of the
position of the Sun due to the angle of the
Earth’s axis of rotation, ~ 23.5 degrees.
Ecliptic – Path of the Sun against the background of the stars.
SCP – South Celestial Pole
NCP -North Celestial Pole
Both directly above the Earth’s Poles.
The Point of Northward Precession –
The point the Sun crosses the Celestial Equator travelling into the Northern Hemisphere. The point is actually moving 1/7 sec/day. It is linked to Celestial Longitude as the point slowly slips along the Celestial Equator.
Epoch For a printed map the stars positions are fixed and so the Epoch date needs to be stated, ie Current Epoch is 2000. Printed star maps are good for about 50 years, then the next epoch data is required for astronomers, ie next epoch will be 2050.
Tropical Year – Time for the Sun to travel from one Northward Equinox to next,
365.2422 days.
Siderial Year – Time taken for the Earth to complete one orbit relative to the fixed stars, 365.2564 days.
The difference between the Tropical and Siderial years is due to the effects of
precession, a gradual westward drift in the ecliptic. The gravitational pull of the Sun and Moon on the Earth’s equatorial bulge cause
the Earth’s axis to trace out a circle on the sky every 25,800yrs.
Judith Bailey – Ballarat Municipal Observatory & Museum
E: info@ballaratobservatory.org.au 0429 199 312
First printed 18.3.2010 Updated 20/7/2022
- 27th Jupiter is at opposition and closest for 59 years. Viewing available at the Observatory Book Online
- 2nd Oct Daylight Savings begins. AEDT Time.
