Lab 1: Latitude, Longitude, Tilt, and Circle of Illumination
Basics (Review)
The Earth rotates on an invisible axis and revolves around the sun. This fundamental motion underpins latitude/longitude and the cycle of day and night.
Reference: YouTube video mentioned for rotation and revolution: http://www.youtube.com/watch?v=taHTA7S JGK
Great Circles and Small Circles
Great circle: An imaginary circle drawn in any direction on Earth’s surface whose plane passes through the center of the Earth. It is the largest possible circle around Earth.
Properties:
Divides Earth into equal halves.
Examples: Hemispheres and Circles of Illumination.
Small circle: Any circle on Earth’s surface that does not divide the Earth into equal halves.
Practical importance: A great circle route is the shortest distance between two points on the Earth’s surface and is often used for long-distance travel.
Latitude
The Equator is a great circle, dividing the Earth into two equal halves: Northern Hemisphere and Southern Hemisphere.
Latitudes are measured north or south of the Equator; reference marks are the North Pole, South Pole, and Equator.
Also known as parallels.
Maximum latitude: 90^\u00b0 (the poles, North or South).
Minimum latitude: 0^\u00b0 (the Equator).
Latitudinal designation uses N/S; e.g., 60° N means 60 degrees north of the Equator.
Longitude
Longitude lines run north and south.
They measure distances east or west of the Prime Meridian (0°) and the International Date Line (180°).
Also known as meridians.
Each line of longitude is half of a great circle.
Longitudes are typically labeled East or West of the Prime Meridian.
International Date Line
Generally follows the 180th meridian.
May shift to accommodate political boundaries (e.g., Alaska vs. Siberia, various Pacific island groups).
Latitude vs. Longitude (grid concept)
Latitude lines run parallel to the Equator; Longitude lines converge toward the poles.
The grid formed by intersecting latitude and longitude lines is used to determine absolute locations (points) on Earth.
Longitude is measured from the Prime Meridian; Latitude from the Equator.
A degree covers a large distance, so the system is subdivided into minutes and seconds:
1^\u00b0 = 60' \,,
Coordinates are typically given in degrees, minutes, and seconds (DMS), with N/S for latitude and E/W for longitude.
Measuring Latitude and Longitude
Start measurement from the Earth’s center.
Latitude measurement is the angle from the Equator toward the poles (0° at the Equator, +/− toward the poles).
Longitude measurement is the angle from the Prime Meridian toward the International Date Line (0° at the Prime Meridian, +/− toward the IDL).
Converting Latitude and Longitude to Decimal Degrees
Start with a coordinate in degrees, minutes, and seconds: e.g., 52^\u00b0 13' 56'' \,N, \, 21^\u00b0 0' 30'' \,E
Decimal degree conversion:
For latitude: 52^\u00b0 \ 13' \ 56'' = 52 + \frac{13}{60} + \frac{56}{3600} \approx 52.23222
For longitude: 21^\u00b0 0' 30'' = 21 + \frac{0}{60} + \frac{30}{3600} = 21.008333
General formula: where D = degrees, M = minutes, S = seconds.
Latitude and Longitude as a Grid System (ranges and conventions)
Latitude ranges from $-90^\u00b0$ (South Pole) to $+90^\u00b0$ (North Pole); North is positive, South negative.
Longitude ranges from $-180^\u00b0$ to $+180^ 0$ or equivalently $0^b0$ to $360^ 0$; East is positive, West negative.
Coordinate system conventions often use N/S and E/W instead of negative signs.
Writing Coordinates and Lab Considerations
Typical format: Latitude first, then Longitude; both in degrees, minutes, and seconds (DMS):
Example: 43^\u00b0 10' 5'' \,N, \, 10^ 0 30' 42'' \,W
For a lab, determine:
Range of degrees on the grid
The value of lines between (i.e., spacing between lines)
Whether the coordinate is North or South (N/S)
Whether the coordinate is East or West (E/W)
Tilt (Inclination) of the Earth
Axial tilt is the 23.5° offset of the Earth’s axis from vertical; the axis is an imaginary line through the North and South Poles around which the Earth rotates.
Four named parallels (latitudes) mark limits of sun’s rays hitting Earth:
Arctic Circle (approx. 66.5° N)
Tropic of Cancer (approx. 23.5° N)
Tropic of Capricorn (approx. 23.5° S)
Antarctic Circle (approx. 66.5° S)
These latitude lines arise directly from the axial tilt and define the maximum tilt positions where the Sun can be directly overhead at noon.
The tilt explains why some latitudes receive more sunlight than others and why daylength changes seasonally.
Tropics and Polar Circles
Tropics: Cancer at 23.5^\u00b0 N and Capricorn at 23.5^\u00b0 S; the Sun’s rays can be vertical (90°) at solar noon within these regions.
Arctic Circle at 66.5^ 0 N and Antarctic Circle at 66.5^ 0 S; these mark the limits of 24-hour daylight and 24-hour darkness at solstices.
Relationship to tilt: the tilt determines the maximum solar declination and thus the extent of the circles of illumination over the year.
Circle of Illumination
Circle of Illumination is a great circle that divides day and night on Earth.
It changes over the year as the Earth orients toward or away from the Sun due to the axial tilt.
It moves from east to west across the globe and is always shifting toward a solstice or an equinox.
Changes in Circle of Illumination (Seasons)
The tilt of the Earth and its orbit around the Sun cause the Circle of Illumination to change over the year.
Key terms: Solstice and Equinox (two of each per year).
Sun’s rays shift between the Tropics and the polar circles across the seasons.
Solstices and Equinoxes
Summer Solstice (approximately June 21):
Vertical rays over the Tropic of Cancer (23.5^\u00b0 N).
Longest daylight of the year in the Northern Hemisphere.
Arctic Circle experiences 24 hours of daylight; Antarctic Circle experiences 24 hours of darkness.
Autumnal Equinox (approximately September 21):
Vertical rays over the Equator at noon.
About 12 hours of daylight and 12 hours of night everywhere between the poles.
Winter Solstice (approximately December 21):
Vertical rays over the Tropic of Capricorn (23.5^\u00b0 S).
Longest daylight of the year in the Southern Hemisphere; Arctic Circle has 24 hours of darkness; Antarctic Circle has 24 hours of daylight.
Spring (Vernal) Equinox (approximately March 21):
Vertical rays over the Equator at noon.
About 12 hours of daylight and 12 hours of night everywhere between the poles.
Practical Lab Considerations (Coordinate Determination)
When determining coordinates for a lab exercise:
Consider the range of degrees on the map grid.
Determine the spacing between lines (i.e., how finely the grid is divided).
Decide whether the location is in the Northern or Southern Hemisphere (N/S).
Decide whether the location is East or West of the Prime Meridian (E/W).
Quick Reference: Key Values and Conversions
Axial tilt: 23.5^\u00b0
Tropics: Tropic of Cancer 23.5^ 0 N, Tropic of Capricorn 23.5^ 0 S
Polar circles: Arctic Circle 66.5^ 0 N, Antarctic Circle 66.5^ 0 S
Circle of Illumination moves east to west and changes with the seasons; tied to solstices and equinoxes.
Decimal degrees from DMS: ; Example conversions:
Latitude: 52^\u00b0 13' 56'' = 52 + \frac{13}{60} + \frac{56}{3600} = 52.23222…
Longitude: 21^ 0 0' 30'' = 21 + \frac{0}{60} + \frac{30}{3600} = 21.008333…
Distance per degree: a degree of latitude is roughly constant in distance, while a degree of longitude varies with latitude (largest at the Equator, zero at the Poles).
Latitude/Longitude signs (convention):
Latitude: North positive (N), South negative (S).
Longitude: East positive (E), West negative (W).
Summary of Relationship Between Features
Latitude and longitude establish a global grid used to locate any point on Earth.
Great circles, including the Equator, are the largest circles and provide the shortest travel routes between two points on the sphere.
The Earth’s axial tilt creates seasons by changing the Circle of Illumination, which causes the Tropics and the polar circles, and sets the pattern of solstices/equinoxes.
The Circle of Illumination moves as the Earth orbits the Sun, producing varying daylengths at different latitudes throughout the year.
Understanding these concepts helps explain daily weather patterns, climate zones, and human activities tied to daylight and seasons.
1 degree = 6/6