🌍 Earth's Coordinates

Earth Science

How do we locate things on Earth?

The problem with landmarks

Directions by landmark

  • "Two miles past the old barn"
  • Depends on knowing the barn
  • Changes when the barn falls down
  • Useless in the middle of an ocean

Directions by coordinate

  • "42°38′ N, 73°45′ W"
  • Works anywhere on the planet
  • Never changes
  • Same answer for every person

A coordinate system gives every point on Earth one unique address built from
a grid of intersecting lines.

Earth's coordinate system

To locate a position on Earth we use a system of two sets of intersecting lines.

Latitude

  • Run east–west, measured north–south
  • Parallel — never touch
  • Measured from the equator (0°)
  • Range: 0° to 90° N or S

Longitude

  • Run north–south, measured east–west
  • Meet at the poles
  • Measured from the prime meridian (0°)
  • Range: 0° to 180° E or W

Latitude lines are parallel; longitude lines are not. Longitude lines converge —
they are farthest apart at the equator and meet at the poles.

Reading the global grid

Line Name Value What it separates
Latitude 0° Equator 0° Northern / Southern Hemisphere
Latitude 90° N North Pole 90° N —
Latitude 90° S South Pole 90° S —
Longitude 0° Prime Meridian 0° Eastern / Western Hemisphere
Longitude 180° International Date Line 180° —

Every coordinate needs a number AND a direction. 43° is not a location.
43° N is a line; 43° N, 76° W is a point.

Degrees and minutes

Between each whole degree there are 60 minutes, written with a prime symbol: ′

Estimating on a map

  • Halfway between degree lines ≈ 30′
  • One-third of the way ≈ 20′
  • We estimate to the nearest 10 minutes

Worked example

Syracuse, NY sits just below 43° N and just past 76° W:

43° N, 76°10′ W

Your turn: read the grid

Give the latitude and longitude of A, B, and C to the nearest 10 minutes.

Latitude and Polaris

How can you find your latitude with your eyes?

Latitude and Polaris

Polaris — the North Star — sits almost directly above Earth's axis of rotation.

Because of that, the altitude (angle above the horizon) of Polaris equals the
observer's latitude.

Polaris is only visible in the Northern Hemisphere.

Moving changes what you see

Travel north

Polaris climbs higher in the sky.
Latitude increases.

Travel south

Polaris drops lower.
Latitude decreases.

Travel east or west

The altitude of Polaris does not change.
Latitude stays the same.

Moving 1° of latitude changes the altitude of Polaris by exactly 1°.
That is about 111 km (69 miles) of travel on the ground.

Longitude and Time

Why does the clock change when you fly west?

Longitude and time zones

Meridians are counted east and west of the prime meridian until they meet at 180°,
the International Date Line.

Time zones are 15° of longitude wide, and each one represents 1 hour of time.

Real time-zone boundaries are jagged, not straight. They bend around state, national,
and island borders for political convenience — the 15° rule is the physics underneath.

Which way does the clock go?

Traveling east

Clocks move ahead (later).
The sun rises there first.

New York → London: +5 hours

Traveling west

Clocks move back (earlier).

New York → Denver: −2 hours

Worked example. Albany is 74° W. Denver is 105° W.

Denver is west of Albany, so Denver's clock is 2 hours behind.

Quick check

1. The angle from the horizon to Polaris is equal to the ________ of the observer.

  • latitude
  • longitude

2. Meridians are 15° apart, so time zones are separated by

  • 4 hours
  • 24 hours
  • 1 hour

3. A ship sails due east across the Atlantic. The altitude of Polaris will

  • increase
  • decrease
  • stay the same

4. How many minutes are in 1° of latitude?

  • 10
  • 60
  • 100

Vocabulary

Latitude — angular distance north or south of the equator, 0° to 90°

Longitude — angular distance east or west of the prime meridian, 0° to 180°

Equator — the 0° line of latitude, halfway between the poles

Prime meridian — the 0° line of longitude, through Greenwich, England

Parallel — another name for a line of latitude

Meridian — another name for a line of longitude

Altitude — the angle of an object above the horizon

International Date Line — the 180° meridian, where the calendar date changes

Exit ticket

1. Give the latitude and longitude of Albany, NY to the nearest 10 minutes
using the ESRT bedrock geology map.

2. An observer measures the altitude of Polaris as 30°. What is their latitude,
and which hemisphere are they in?

3. Explain, in one sentence, why time zones are 15° of longitude wide.

LESSON GOAL: Students can state and locate positions on Earth using latitude and longitude in degrees and minutes, explain the Polaris–latitude relationship, and connect longitude to time zones. MATERIALS: 2024 ESRT (Generalized Bedrock Geology of New York State, pp. 2-3), guided notes packet, projected world grid. TIMING: One 40-minute period, or the first half of an 80-minute block with the practice set as the back half. OPENER: Put the driving question on the board and take 60 seconds of turn-and-talk. "If you had to tell someone exactly where you are standing right now — and they had no phone, no GPS, no address — what would you tell them?" Collect two or three responses. Students usually reach for landmarks or distances from a known place. That is exactly what a coordinate system replaces.

TEACHER MOVE: Run this as a fast contrast, not a lecture. Two minutes maximum. EXPECTED STUDENT RESPONSES: Students often propose an address or a GPS pin. Push on both — an address requires a street grid that exists only where people live, and a GPS pin is *already* latitude and longitude, which is the point of the lesson. KEY POINT TO SURFACE: The grid has to be built from something that does not move. That is why the system is anchored to Earth's rotation: the poles and the equator come from the axis of rotation, not from human decisions. The prime meridian is the one piece that *was* a human decision (Greenwich, 1884) — worth naming, because students ask. TRANSITION: "So let's build that grid."

TEACHER MOVE: Build this on the board as you say it rather than revealing the whole slide. Draw the circle, mark the axis, then the equator, then two parallels, then the meridians converging at the poles. COMMON MISCONCEPTION #1: "Latitude lines run north-south because they measure north-south." The line runs east-west; the *measurement* is north-south. Give students the mnemonic they will actually keep: latitude lines are flat, like the rungs of a ladder. COMMON MISCONCEPTION #2: Students expect latitude to go to 180° like longitude. Ask why it stops at 90°: because you have run out of Earth — you are at the pole. Longitude goes to 180° because you can travel halfway around in either direction. CHECK: "Which set of lines are parallel?" (latitude) "Which set meets at a point?" (longitude) Cold-call two students.

TEACHER MOVE: Project a world grid map (the Nearpod grid map, or the ESRT NYS map for the regional version). Have students put a finger on the equator, then the prime meridian, then the point where they cross (0°, 0° — in the Gulf of Guinea, off West Africa). CONFERRING QUESTION while circulating: "What hemisphere are we in? How do you know from the map, not from memory?" KEY POINT TO SURFACE: Latitude is always written first, then longitude. This is the single most common formatting error on the Regents. Students who write "76° W, 43° N" have located a point in the middle of nowhere or nowhere at all. DIFFERENTIATION: For students who need it, the hemisphere check is mechanical — above the equator is N, below is S; left of the prime meridian (in the Western Hemisphere) is W, right is E.

TEACHER MOVE: This is the procedural heart of the lesson and where students lose points on the Regents. Model the estimation out loud on a projected ESRT: "I need Syracuse. It's sitting right on the 43° parallel, so latitude is 43° N. For longitude, it's a little past the 76° meridian, maybe a sixth of the way, so about 10 minutes — 76°10′ W." COMMON MISCONCEPTION: Students treat minutes as decimals. 76.5° is NOT 76°50′ — it is 76°30′. Do this one explicitly; it catches a third of the class. Half a degree is thirty minutes because an hour analogy holds: 30 minutes is half an hour. ANALOGY THAT WORKS: A clock. Sixty minutes in a degree, sixty minutes in an hour. Half past is :30 on both. EXPECTED STUDENT RESPONSES: Some will ask about seconds. Yes — 60 seconds in a minute — but the Regents only requires degrees and minutes, estimated to the nearest 10′. TIMING: 6-8 minutes including the worked example.

TEACHER MOVE: Silent independent work for 3 minutes, then partner check, then reveal. Students record these in the guided notes. ANSWERS: A = 43°00′ N, 76°30′ W (Syracuse area) B = 44°30′ N, 73°00′ W (Plattsburgh / Lake Champlain area) C = 42°50′ N, 78°50′ W (Buffalo area) LOOK FOR: Students who reverse latitude and longitude, drop the hemisphere letter, or write 76.5° W instead of 76°30′ W. All three are the errors that cost points. CONFERRING QUESTION: "How did you decide on the minutes?" You want to hear fraction-of-the-gap reasoning, not guessing. EXTENSION for early finishers: "These are three real New York cities. Find them on the ESRT bedrock map and name them." TRANSITION: "Latitude and longitude are both just angles. Here's how sailors measured one of them for four hundred years, with no instruments but a stick."

SEGMENT GOAL: Students explain why the altitude of Polaris equals the observer's latitude, and predict how that altitude changes as an observer moves. TIMING: 10 minutes. HOOK: "You are on a ship in 1650. No GPS, no radio, no landmarks — just ocean. You can still find your latitude to within a degree. How?"

TEACHER MOVE: Work the geometry with the diagram. The line of sight to Polaris is effectively parallel to Earth's axis because Polaris is so far away. The horizon is tangent to Earth at the observer. Those two facts force the altitude angle to equal the latitude angle. CHECK THE EXTREMES — this is the move that makes it stick: At the North Pole (90° N), Polaris is directly overhead — altitude 90°. ✅ At the equator (0° N), Polaris sits on the horizon — altitude 0°. ✅ In the Southern Hemisphere, Polaris is below the horizon — you cannot see it. ✅ Walk all three. Students who can reconstruct the extremes never forget the rule. COMMON MISCONCEPTION: "Polaris is the brightest star." It is not — it is about 48th. It matters because of *where* it is, not how bright. SECOND MISCONCEPTION: Students think Polaris has always been and will always be the pole star. Earth's axis precesses on a ~26,000-year cycle; Vega gets a turn. Mention only if you have time — it is enrichment, not Regents content. CONNECTION: Ties directly to the ESRT latitude scale and to any Regents question showing an observer sighting Polaris with a protractor or astrolabe.

TEACHER MOVE: Quick-fire prediction round. "I'm in Albany and Polaris is at 42°. I drive to Montreal. Higher or lower?" (Higher — 45°.) "I drive to Buffalo." (Same — Buffalo and Albany are nearly the same latitude, ~42-43°.) "I fly to Miami." (Lower — about 26°.) WHY BUFFALO MATTERS: It is the east-west case, and it is the one students miss. New York State is wide enough east to west that students assume something must change. Latitude does not. Longitude does. REGENTS LINK: Questions of the form "An observer travels from X to Y; how does the altitude of Polaris change?" appear on nearly every exam. The whole skill is deciding whether the motion had a north-south component. FORMATIVE CHECK: Thumbs up / down / sideways for higher / lower / same on three more scenarios of your choosing.

SEGMENT GOAL: Students connect Earth's rotation rate to the 15°-per-hour spacing of time zones and compute time differences from longitude differences. TIMING: 10-12 minutes.

TEACHER MOVE: Derive the 15° rather than announcing it. "Earth turns all the way around — 360° — in how long? So how far does it turn in one hour?" Let a student do the division out loud. KEY POINT TO SURFACE: This is a direct consequence of rotation. The same rotation that gives us day and night gives us time zones. It is not an arbitrary convention; the *boundaries* are conventional, the *spacing* is not. EXPECTED STUDENT RESPONSES: Someone will bring up daylight saving time, or the fact that China uses one time zone for the whole country. Both are true and both are political decisions layered on top of the physics. Acknowledge and move on. CONFERRING QUESTION: "If a place is 45° of longitude east of us, how many hours different is its clock?" (3 hours — and ahead, not behind.)

TEACHER MOVE: Anchor the direction with rotation, not memorization. Earth rotates west to east, so eastern locations rotate into sunlight first — their clocks are ahead. Demonstrate with a globe and a lamp if you have one out; a fist and the projector beam works too. COMMON MISCONCEPTION: Students reverse it, reasoning that "east is ahead on the map." That happens to give the right answer, but for the wrong reason, and it collapses on the date line. Push for the rotation explanation. WORK THE MATH: 31 ÷ 15 = 2.07, which rounds to 2 hours. Note for students that the answer is approximate because Denver's actual zone boundary is not exactly at a multiple of 15°. DIFFERENTIATION: Struggling students can use the two-step recipe — (1) subtract the longitudes, (2) divide by 15, (3) decide ahead or behind by direction. Post it. EXTENSION: "What happens when you cross the International Date Line?" You change the *date*, not just the hour — westbound you lose a day, eastbound you gain one.

ANSWERS: 1 — latitude. 2 — 1 hour. 3 — stay the same. 4 — 60. TEACHER MOVE: Run as whiteboards or a Nearpod poll. All four at once, 90 seconds, then reveal one at a time and ask for reasoning on any item where the class split. LISTEN FOR on #3: "Sailing east is an east-west motion, and east-west motion doesn't change latitude, and Polaris tracks latitude." That full chain is the target response. Accept "latitude stays the same" but push one student to give the reason. IF #2 SPLITS: Re-derive 360 ÷ 24 on the board. If students chose 4 hours, they may have divided 60 by 15 — a plausible wrong move worth naming out loud.

TEACHER MOVE: Do not read this slide aloud. It is a reference page for the notes and for studying. Point students to it and move on. DIFFERENTIATION: Students who need vocabulary support should have these eight terms pre-printed in their guided notes rather than copying them down. STUDY NOTE for students: The pairs matter — parallel/latitude and meridian/longitude. Regents questions use the formal terms interchangeably with the common ones.

ANSWERS: 1. Albany ≈ 42°40′ N, 73°45′ W. Accept 42°40′-42°50′ N and 73°40′-73°50′ W. 2. 30° N latitude, Northern Hemisphere. The hemisphere is forced — Polaris is not visible from the Southern Hemisphere at all. 3. Earth rotates 360° in 24 hours, which is 15° per hour, so each 15° of longitude corresponds to one hour of time. TEACHER MOVE: Collect as students leave. Question 2 is the diagnostic one — if students give the latitude but miss the hemisphere, they have memorized the rule without understanding where Polaris can be seen from. NEXT LESSON: Elevation and topographic maps — the third coordinate. We can now locate any point on the surface; next we add how far above sea level it sits.