الأسبوع 26 · المحور الرابع «التغير والثبات»، الوحدة الرابعة «الأنماط في السماء»، المفهوم 4.2 «أنماط الحركة في السماء» (أ): الليل والنهار والظلال · 3 دروس
2
Sunrise in Marsa Alam and Siwa
Discovery Education Science P5: Term 2, Unit 4, pp. 138–141
GoalRead sunrise times in two places, find patterns, and explain day length and apparent motion with Earth’s steady tilt.
New words
sunrise · the time when the top of the Sun first appears over the horizontilt · the slant of Earth’s axis, about 23½°, always pointing the same waydaylight hours · the time between sunrise and sunset
Explanation
The Sun does not rise at the same moment everywhere. Marsa Alam is on the Red Sea, in the east of Egypt, and Siwa is an oasis in the west. Earth turns toward the east, so places in the east meet the sunlight first. On a winter morning the Sun might rise in Marsa Alam at about 6:10 and in Siwa at about 6:50, which is 40 minutes later. Sunrise time also changes slowly through the year. This is because Earth’s axis is tilted, about 23½° away from straight up, and it keeps pointing the same way in space as Earth goes round the Sun. During part of the year Egypt leans toward the Sun, and the days are long and warm. Half a year later it leans away, and the days are short and cooler. The tilt does not change from month to month. What changes is which part of Earth leans toward the Sun.
Worked examples
Place
Sunrise (example)
Marsa Alam (east)
6:10
Siwa (west)
6:50
Example sunrise times on one winter morning: Marsa Alam 6:10, Siwa 6:50. The difference is 40 minutes. Sunset in Marsa Alam is also earlier than in Siwa, because the east always meets the Sun’s light first and leaves it first.
Daylight in Egypt is about 10 hours in midwinter and about 14 hours in midsummer. The difference is 14 − 10 = 4 hours. In the graph, the longest days come around June, when Egypt leans toward the Sun, and the shortest around December.
What stays the same
What changes
tilt of about 23½°
Earth’s place on its orbit
the way the axis points
which end leans toward the Sun
A common mistake says, “Earth’s tilt changes through the year.” The tilt stays the same. What changes is where Earth is on its orbit, so a different end of the axis leans toward the Sun. That makes the seasons.
Class activity
Sunrise detective (a grown-up helps). Look up (or ask a grown-up to look up) the sunrise times in your town for the first day of three different months, and write them in a table. Which month has the earliest sunrise? Which has the latest? Do not look at the Sun. On a globe, find Marsa Alam and Siwa and say which is farther east.
Practice
1
Why does the Sun rise earlier in Marsa Alam than in Siwa?
Marsa Alam is farther east, and Earth turns toward the east, so it meets sunlight first.
2
Sunrise is at 6:20 in the east and 6:55 in the west. How many minutes later is it in the west?
Place
Sunrise
east
6:20
west
6:55
6:55 − 6:20 = 35 minutes later.
3
Look at the graph. In which month are the days longest, and how much longer than in January?
June, with about 14 hours, which is 14 − 10 = 4 hours more than January.
4
True or false: Earth’s tilt changes from 23½° to 0° and back each year.
False. The tilt stays about 23½° and points the same way. What changes is which end leans toward the Sun.
