All about the great conjunction of Jupiter and Saturn on December 21, 2020

1. The December 21, 2020
“Great Conjunction” of
Jupiter and Saturn.

2. What is “Great Conjunction” ?
 A “great conjunction” is a conjunction of the
planets Jupiter and Saturn.
 Great conjunctions occur regularly, about every 19
years and 7 months, due to the combined effect of
Jupiter's approximately 11.86-year orbital period and
Saturn's approximately 29.5-year orbital period.

3. What is “Great Conjunction” ? (cont..)
 As it travels around the Sun, Jupiter catches and moves past slower orbiting Saturn – in a somewhat
regular 20-year pattern.
 Jupiter and Saturn last aligned in a great conjunction in May 2000.
 After this December 21, 2020 conjunction, the two planets will appear to trade positions, as Jupiter
overtakes Saturn. Jupiter will progress to the east and draw farther and farther away from Saturn
throughout the decade of the 2020s.

4. What is “Great Conjunction” ? (cont..)
 The two planets will
approach each other
through the 2030s, reaching
conjunction again in
November 2040. Beyond
that, Jupiter-Saturn
conjunctions will occur in
April 2060, March 2080, and
September 2100.

5. Why This “Great Conjunction” is Very
Important?
 In most cases the separation between Jupiter and Saturn is about 1-2 degrees.
 On December 21, the day of the conjunction, Jupiter and Saturn will be separated by a mere
0.1 degrees, and may appear as a single bright “star.” And It will be the closest great
conjunction since July 16, 1623.
 In fact, the 2020 great conjunction of Jupiter and Saturn is exceptionally close. Over a period
of one thousand years, from 1600 to 2599, there are only six great conjunctions where the
minimum separation between Jupiter and Saturn is less than 0.2 degrees: 1623, 1683, 2020,
2080, 2417, and 2477.

6. What Can We See, and When Can We See
It?
 Since September 2020, Jupiter has been moving closer and closer to Saturn in the early evening sky.
 Wherever you are in the world, even in light-polluted urban environments, the two planets are an
impressive sight, and easy to find after sunset.
 If you're in the Northern Hemisphere, look toward the southwest. If you're in the Southern Hemisphere,
look toward the west. Jupiter is the brighter of the pair.
 As November begins, Jupiter and Saturn are five degrees apart, which is the width of your three middle
fingers held at arm's length.

7. What Can We See, and When Can We See
It?
 At the start of December, the planets
will be two degrees apart, and still
moving closer together.
 A few days before December 21, a thin
Crescent Moon will pass close to Jupiter
and Saturn in the sky. In the Western
Hemisphere, the Moon will be closest to
the two planets on December 16. In the
Eastern Hemisphere, it will be closest on
December 17.

8. How Can We see It?
 Anyone who has access to a telescope or
even a moderate pair of binoculars can
observe the great conjunction.

9. How To Find The Distance Between Them?
 We measure the distance between celestial objects based on the angle they make with an
observational point on Earth. Known as angular distances or angular separation, distances are
expressed in terms of degrees (°), arc minutes ('), and arc seconds (").
 While angular separation primarily describes the apparent distance between celestial objects, as
seen from Earth, it can also be used to suggest their actual distance from one another.

10. How To Find The Angular Separation?
 Astronomers used the Cross-Staff
for measuring the angle between the
directions of two stars.
 Other, older instruments for this
purpose existed, used by scholars
such as Hipparchus and Ptolemy, but
none was as portable, which made
the cross-staff eminently suitable for
navigation at sea.

11. How To Find The Angular Separation?
 To measure the angle between two stars, an
astronomer would place the staff just below one
eye (drawing) and slide the cross-piece up and
down.
 The cross-piece would have a pair of open sights
sticking out perpendicular to the drawing at
symmetric locations such as B and B’.
 The astronomer would slide the cross-piece up and
down, until sight B covered one of the stars and
sight B' the other. For use at night, slits make
convenient sights.

12. How To Find The Angular Separation?

13. How To Find The Angular Separation?
 After that was achieved, the instrument would be lowered and the distance AC would be measured.
 Then if A was the angle between the staff and the direction of one star, from the definition of the
tangent
TanA = BC/AC
 The distance BC between the sight and the stick was already known to the astronomer so, using a
table of tangents, the angle denoted by A could be calculated. Since the instrument was symmetric,
the angle between the directions of the stars was 2A.

14. Graph of Angular Separation

15. Thank You
Hariprasad M and Harsha C
Presented by,