## Meridian

Recall that stars cross the observer's meridian and culminate when they are due south Use star data and charts to determine the time at which a star will cross the observer's meridian.

The observer's meridian is the line between north and south through the observer. Culmination is when a star is at the highest point in the sky (which should be along the meridian).

##### Question 1

The star Betelgeuse rises from London at 20.03 on the 23rd November. What time will it rise on 30 November? Hint: Stars rise approx 4 minutes earlier each day.

23/11 to 30/11 = 7 days
7 x 4 = 28
20.03 - 00.28 = 19.35 so Betelgeuse rises from London at 19.35 on 30 November.

##### Question 2

Betelgeuse has a Right Ascension of 05h 55m. It culminates at 02.03 on 6th January. Rigel culminates at 01.23 on 6th January. What is Rigel’s right ascension?

01.23 minus 02.03 = Difference of -40 minutes
05.55 – 40 minutes = 05.15m = Right Ascension of Rigel

##### Question 3

On the same date Sirius has a Right Ascension of 06h 45m. What time does it culminate?

06.45 – 05.55 = Difference of +50 minutes
02.03 + 50 minutes = 02.53

##### Question 4

The star Betelgeuse rises from London at 20.03 on the 23rd November. A person 3°
west sees the star culminate at what time?

1° West = difference of +4 minutes
3° = 12 minutes.
Star culminates at 20.03 + 0.12 = 20.15

##### Question 5

Assume Polaris has a declination of 90° North. At what angle in the sky would it be visible from these locations?
a) Beijing 40°N
b) Bombay 19°N
c) Wellington 41°S

a) 40°
b) 19°
c) It wouldn’t be visible. It would be 41° below the horizon when culminating.

##### Question 6

A star seen from London (51°) culminates at 30° above the horizon. What is the star’s declination?

90 – 51 = 39
30-39= 9° N

##### Question 7

A star culminates at 50°. It has a declination of +20°. What is the latitude from where it is observed?

50 - 30 = 20°
Latitude = 20°

##### Question 8

A person on the Tropic of Capricorn measures the angle of the Sun at midday on different days. What angle is it at on these dates?
a) March 21st
b) June 21st
c) September 21st
d) December 21st

a) & c) 66.5°
b) 47°
d) 90°

##### Question 9

It is midday on December 21st. What angle is the Sun in the sky at these locations?
a) The Tropic of Cancer (23.5° North)
b) Sydney (34° South)
c) South Pole (90° South)
d) London (51° North)
Clue: The Sun is 66.5° above the Equator on this date, 23.5 degrees from the zenith.

a) 90 – 23.5 – 23.5 = 47
b) 90 – 34 + 23.5 = 79.5*
c) 90 – 90 + 23.5 = 23.5*
d) 90 - 51 -23.5 = 15.5

##### Question 10

Two observers, Dan and Boski observe the star Capella at different places. Dan sees Capella cross the meridian at 19.00. Boski is 3° east of Dan.
a) What time does Boski see Capella cross the meridian?
b) A year later the same happens only now, Boski sees Capella 10 minutes later than Dan. How far east or west has Boski gone?

a) 1° = 4 minutes difference. 3° = 12 minutes. East means the number is negative and so sees it sooner.
19.00 minus 12 minutes = 18.48
b) 4 minutes difference = 1°. 10 minutes = 2.5° difference. Seeing the star later means he is now west so he is 2.5° west of Dan.

>
###### Did you know?

Technically any point on Earth is on a meridian between north and south poles.