**Yr. 11 Physics - AstronomySun Observational Activity•**Local Midday & Latitude• Finding True North/South**Sun**Observing The Sun’s Motion On a sunny day a stick, called a gnomon, placed vertically into the ground will cast a shadow. gnomon shadow The movement of the gnomon’s shadow can be used to: • track the Sun’s passage across the sky • determine local midday • find True North and South • determine the latitude of your location**From sunrise in the morning the length of the shadow cast by**the gnomon gets shorter until at midday the shadow is at its shortest. Sun Due to the tilt in the Earth’s axis the length of the midday shadow changes throughout the year. It is shortest, in the Southern Hemisphere, on December 21st - the Summer Solstice. gnomon 11.00 am shadow 12.00 pm 1.30 pm paper Conversely, the gnomon’s midday shadow will be longest on June 21st - the Winter Solstice.**South Celestial Pole**Celestial Equator gnomon shadow True South Summer Solstice - Midday December 21st The Summer Solstice marks the day of the year with the most hours of daylight and the gnomon’s shadow will be at its shortest for the year at midday.**South Celestial Pole**Celestial Equator gnomon shadow True South Autumn Equinox - Midday March 22nd At the Autumn and Spring Equinoxes the hours of daylight and night are equal in length.**South Celestial Pole**Celestial Equator gnomon shadow True South Winter Solstice - Midday June 21st The Winter Solstice marks the day of the year with the least hours of daylight and the gnomon’s midday shadow will be at its longest for the year.**South Celestial Pole**Celestial Equator gnomon shadow True South Spring Equinox - Midday September 21st At the Autumn and Spring Equinoxes the hours of daylight and night are equal in length.**Sun**gnomon shadows paper True South Local Midday & True North South By recording the position of the gnomon’s shadow at regular intervals a relatively accurate determination of the time of local midday can be obtained when the shadow is at its shortest. Given that the Sun appears in the Northern part of our sky it follows that at local midday the shadow cast by the gnomon will point True South.**(**) gnomon height q1 = tan-1 shadow length Local Latitude (Summer Calculation) South Celestial Pole Celestial Equator gnomon q1 shadow Step 1 Calculate q1, the angle of elevation between the shadow’s end and the top of the gnomon.**South Celestial Pole**Celestial Equator q2 gnomon shadow Step 2 Determine q2, the angle of the Sun’s declination. This is the Sun’s angular distance from the Celestial Equator, it can be obtained from a book containing astronomical data. q2 = Sun’s Declination Angle**South Celestial Pole**Celestial Equator q2 gnomon q1 q3 shadow Step 3 Calculate q3, the angle of elevation between the horizon and the South Celestial Pole. q3 corresponds to your local latitude. q3 = 180o - 90o - (q1 - q2) or q3 = 90o - (q1 - q2)**)**( q1 = tan-1 15.7 3.9 Sample Summer Calculation Place: Barjarg, Victoria Date: December 27th 2003 Sun’s Declination: 23.32o South Gnomon Height: 15.7 cm Shadow Length: 3.9 cm q3 = 90o - (q1 - q2) q3 = 90o - (76.05o- 23.32o) q3 = 37.27o So Barjarg’s Latitude is 37.27o South. q1 = 76.05o**South Celestial Pole**Celestial Equator gnomon q1 shadow ( ) gnomon height q1 = tan-1 shadow length Local Latitude (Winter Calculation) Step 1 Calculate q1, the angle of elevation between the shadow’s end and the top of the gnomon.**South Celestial Pole**Celestial Equator q2 gnomon shadow Step 2 Determine q2, the angle of the Sun’s declination. This is the Sun’s angular distance from the Celestial Equator, it can be obtained from a book containing astronomical data. q2 = Sun’s Declination Angle**South Celestial Pole**Celestial Equator q2 gnomon q3 q1 shadow Step 3 Calculate q3, the angle of elevation between the horizon and the South Celestial Pole. q3 corresponds to your local latitude. q3 = 180o - 90o - (q1 + q2) or q3 = 90o - (q1 + q2)**)**( q1 = tan-1 14.0 24.6 Sample Winter Calculation Place: Stanley, Tasmania Date: May 18th 2003 Sun’s Declination: 19.57o North Gnomon Height: 14.0 cm Shadow Length: 24.6 cm q3 = 90o - (q1 + q2) q3 = 90o - (29.64o+ 19.57o) q3 = 40.79o So Stanley’s latitude is 40.79o South. q1 = 29.64o