Graphing the Patterns of Day and Night

The Phenomenon

You also notice that your friend who lives in Alaska tells you their summer days last almost 20 hours, while their winter days have fewer than 6 hours of daylight. Meanwhile, your cousin near the equator in Ecuador says their days are nearly the same length all year round — about 12 hours of light and 12 hours of darkness.

What is going on? Why don't all places on Earth experience the same amount of daylight? And why does the pattern repeat so predictably that scientists can tell you the exact minute of sunrise and sunset months in advance?

• Why do you think the amount of daylight is so different in Alaska compared to Ecuador?
• What patterns might you find if you graphed the hours of daylight for each month?
• What could you measure or track to help explain these differences?

What Scientists Know

To understand why day and night follow predictable patterns, scientists study two important motions: Earth's rotation on its axis and its revolution around the Sun. These two motions work together to create the regular cycles of daylight and darkness that we can observe, measure, and graph as data.

Let's Investigate

Graphing Daylight Data for Two Cities

Scientists use a key practice called analyzing and interpreting data. In this investigation, you will examine real daylight data for two different locations and create line graphs to discover patterns.

Question: How does the number of daylight hours change throughout the year in different locations?

Materials you would use:

• Daylight data table (hours of daylight per month for two cities)
• Graph paper or graphing software
• Colored pencils (one color per city)
• Ruler for drawing straight axes

Procedure: Plot the months (January–December) on the x-axis and hours of daylight (0–24) on the y-axis. For each city, plot one data point per month and connect the dots with a smooth curve. Then compare the two lines to identify patterns.

Investigation Data: Average Daylight Hours per Month

Daylight Hours Throughout the Year — Line Graph

What We Discovered

When we look at the graph, several important patterns jump out right away. The line for Quito, Ecuador is almost completely flat — it barely changes all year, staying right around 12 hours of daylight every single month. The line for New York makes a gentle wave shape, rising to about 15 hours in June and dipping to about 9 hours in December. And the line for Anchorage, Alaska makes the most dramatic wave of all, swinging from less than 6 hours of daylight in December to nearly 20 hours in June.

These differences all come back to Earth's tilted axis. Quito sits right on the equator (0° latitude), so it always receives roughly equal amounts of sunlight and darkness regardless of where Earth is in its orbit. New York is at about 40° north latitude, so it's tilted toward the Sun in summer (more daylight) and away from the Sun in winter (less daylight). Anchorage, at 61° north, is much closer to the North Pole, so the tilt effect is even more extreme.

Notice something else: all three locations reach their maximum daylight at the same time of year (around June) and their minimum at the same time (around December). This tells us the pattern is driven by Earth's position in its orbit, not by something local to each city. The shape of each line is similar — a smooth wave — but the amplitude (how far the wave swings up and down) depends on latitude.

The data also reveals something powerful: this pattern is predictable and repeating. If you collected daylight data for 2023, 2024, and 2025 and plotted them on the same graph, the three lines would overlap almost perfectly. This regularity is what allows almanacs to publish sunrise and sunset times years in advance. The pattern repeats because Earth's orbit and tilt don't change from year to year (at least not on human timescales).

This is exactly why scientists use graphs rather than just tables of numbers. A table tells you the data, but a graph shows the pattern. When you look at the line graph, you can instantly see the wave shape, compare the three cities, spot the peaks and valleys, and even estimate daylight hours for months that aren't in the data. Graphs turn raw numbers into visual stories.

Patterns and Connections

The crosscutting concept at work in this lesson is Patterns. Scientists look for patterns in data to help explain and predict what will happen. In our daylight data, the pattern is a regular, repeating cycle — daylight hours increase from winter to summer, then decrease from summer to winter, and this cycle repeats every 12 months. Recognizing this pattern allows scientists to make accurate predictions about future daylight hours.

The fascinating thing about patterns is that they appear everywhere in science, not just in day-and-night data. Let's look at how the same kind of repeating pattern shows up in other areas:

Every one of these patterns can be graphed. When we graph repeating patterns, we see that distinctive wave shape — scientists call this a cyclical pattern. The specific numbers change depending on what we're measuring, but the shape of the graph (a rising and falling curve that repeats) is strikingly similar. This tells us that the underlying causes share something in common: they all involve objects (Earth, Moon) moving in regular orbits or rotations.

Real-World Connections & Engineering

Understanding day-and-night patterns isn't just interesting science — it's information that people use every day to solve real-world problems. Here are some examples of how graphing daylight data matters outside the classroom:

In each of these cases, people are doing what scientists do: collecting data, graphing it to find patterns, and then using those patterns to make decisions and solve problems. The science practice of analyzing and interpreting data isn't just for the classroom — it's a skill that engineers, farmers, city planners, and many other professionals rely on daily.

Key Vocabulary Review

• Rotation — The spinning of Earth on its axis, which takes about 24 hours to complete. Rotation is what causes day and night.
• Revolution — The movement of Earth in its orbit around the Sun, which takes about 365.25 days (one year) to complete.
• Axis — An imaginary line running through Earth from the North Pole to the South Pole. Earth is tilted on its axis at about 23.5 degrees.
• Latitude — A measurement that describes how far north or south a location is from the equator. Latitude affects how many hours of daylight a place receives during different seasons.
• Pattern — A regular, repeating arrangement in data that scientists can use to describe what has happened and predict what will happen in the future.
• Cyclical pattern — A pattern that repeats in a regular cycle, like daylight hours rising and falling over the course of a year.
• Line graph — A type of graph that connects data points with a line to show how a measurement changes over time. Line graphs are especially useful for revealing patterns.
• Data — Facts and numbers collected through observation or measurement that can be analyzed and interpreted to answer scientific questions.

Practice: Test Your Understanding

What's Next?