1.2 Using the Earth Science Reference Tables (ESRT)
Key Takeaways
- Gradient = change in field value / distance; it measures how quickly a field (elevation, temperature, pressure) changes over a map area.
- Density = mass / volume is constant for a material regardless of sample size; it is the key to flotation, layering, and mineral identification.
- Eccentricity = distance between foci / length of major axis; a more circular orbit has a smaller eccentricity, closer to 0.
- Rate of change = change in field value / change in time tells how fast a quantity (temperature, position, water level) changes.
- The Generalized Landscape, rock-identification charts, Solar System Data, and Geologic History of New York tables turn the ESRT into a reasoning tool, not just a fact sheet.
Using the Earth Science Reference Tables (ESRT)
Quick Answer: The 2024 Earth and Space Sciences Reference Tables (ESRT) are provided during the exam and contain the equations and charts you are expected to use. Master four formulas - gradient, density, eccentricity, and rate of change - and learn to read the Generalized Landscape, rock-identification, Solar System Data, and Geologic History of New York charts. Reference-table fluency is one of the highest-yield skills on the test.
The ESRT is open during the whole written exam. It is not a crutch handed out to weak students - it is a designed reasoning tool. Many questions are unanswerable without it, and many more are fast only if you know exactly which page to flip to. Treat the tables like a calculator you must practice operating.
The Four High-Yield Equations
These four equations appear on the ESRT equations page. Memorize what each one means, not just its letters.
| Quantity | Formula | What it tells you |
|---|---|---|
| Gradient | change in field value / distance | How steeply a field (elevation, temperature, pressure) changes across a map |
| Density | mass / volume | How much matter is packed into a volume; a fixed property of a material |
| Eccentricity | distance between foci / length of major axis | How elongated an elliptical orbit is (0 = circle) |
| Rate of change | change in field value / change in time | How fast something changes over time |
Gradient
Gradient = change in field value / distance. Suppose two points on a topographic map are 40 km apart, one at 300 m elevation and one at 180 m. The gradient is (300 - 180) / 40 = 3 m/km. Closely spaced isolines mean a steep gradient; widely spaced isolines mean a gentle gradient. The same equation works for a temperature field or an air-pressure field.
Density
Density = mass / volume. A sample with 150 g and 50 cm3 has a density of 150 / 50 = 3 g/cm3. Density is independent of sample size - breaking a rock in half does not change its density. Density controls whether material floats or sinks and why Earth's denser materials settled to form the core.
Eccentricity
Eccentricity = distance between foci / length of major axis. It describes how stretched an ellipse (an orbit) is. A value of 0 is a perfect circle; values approach 1 for a very flattened ellipse. If an orbit's foci are 3 cm apart and the major axis is 12 cm long, eccentricity = 3 / 12 = 0.25. Earth's orbit has a low eccentricity (about 0.017), so it is nearly circular. Always divide the smaller number (foci distance) by the larger number (major axis), so the answer is between 0 and 1.
Rate of Change
Rate of change = change in field value / change in time. If a stream's water level rises from 2 m to 6 m over 8 hours, the rate of change is (6 - 2) / 8 = 0.5 m/hr. This formula handles cooling temperatures, rising tides, moving glaciers, and radioactive decay framing on the exam.
Reading the ESRT Charts
Beyond equations, several charts do heavy lifting:
- Generalized Landscape Regions of New York State - shows whether an area is a plateau, lowland, plain, highland, or mountain region. Steep, dissected uplands appear as plateaus/mountains; flat areas as lowlands/plains. Use it to connect bedrock and elevation to landscape type.
- Rock-identification charts (igneous, sedimentary, metamorphic, and the Scheme for Igneous Rocks) - read across rows for grain size, composition, texture, and environment of formation. The Scheme for Igneous Rocks links color/density to mineral composition (felsic = lighter/less dense; mafic = darker/denser).
- Solar System Data - gives each planet's mass, diameter, density, period of revolution and rotation, distance from the Sun, and eccentricity. Use it for comparisons and for eccentricity questions.
- Geologic History of New York State - the time chart of eras, periods, life forms, index fossils, and major NY events (mountain-building, glaciation). Use it for relative-age and index-fossil reasoning.
A Reference-Table Workflow
Strong test-takers do not hunt blindly. Build a mental index so each question type maps to a page:
- A question with mass and volume, or asking why something sinks or floats, points to the density equation.
- A question with two map points and a distance points to gradient.
- A question about an orbit's shape points to eccentricity and the Solar System Data table.
- A question about how fast something changes over time points to rate of change.
- A rock photo or description points to the rock-identification charts.
- A relative-age or fossil question points to Geologic History of New York State.
Practice the equations until the arithmetic is automatic, because clusters often ask you to calculate, then interpret the result in the next question. A correct number with the wrong units can still lose constructed-response credit, so carry units through every step. The more you rehearse with the actual 2024 ESRT in hand, the faster and more accurate you become on test day.
Two weather stations are 200 km apart. One reports an air pressure of 1016 millibars and the other 1004 millibars. What is the pressure gradient between them?
On the ESRT, an elliptical orbit is drawn with its two foci 2 cm apart and a major axis 10 cm long. What is its eccentricity, and what does the value indicate?