T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 1

Name: Tides Unit I: Observations (4 pts)

Section:

What are tides?

Tides are the slow, daily rise and fall of the ocean surface over a large area caused (primarily) by the gravitational pull of the Moon. In most places, there are 2 high tides and 2 low tides each day (24 hours), so it takes about 6 hours for the ocean to go from high tide to low tide.

1. What are tides?

2. How many high tides are there each day?

3. How many low tides are there each day?

4. About how long does it take for the ocean to go from high tide to low tide?

5. What is the primary cause of the tides?

T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 2

Reading Tide Charts

Tides are typically described using tide charts. Reading tide charts is fairly straightforward. The high points (the peaks or crests) in the curve indicate high tides, and low points (the valleys or troughs) indicate low tides. The chart below shows high tides at 3 a.m. and 3 p.m., and low tides at 9 a.m. and 9 p.m. Sea level rises to 5 feet during the 1st high tide, and 7 feet during the second. It drops to 1 foot during the 1st low tide, and 2.5 feet during the second. Sea level measurements are typically made relative to mean low low water (MLLW), the average of the lowest daily low tide over a long period of time. Thus, a high tide with a height of 4 feet is 4 feet above the typical lowest-low-tide-of-the-day.

The difference in height between successive high and low tides is called the tidal range. In the chart shown above, the largest tidal range is the difference in height between the 1st low tide (height of 1 foot) and the 2nd high tide (height of 7 feet). So, the largest tidal range of the day is 6 feet (7 feet – 1 foot).

T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 3

6. Use the tide chart below to determine the height and time of the high tides and low tides on December 8, 2004, at the Port of Los Angeles.

1st High Tide 1st Low Tide 2nd High Tide 2nd Low Tide

Time

Height

7. What was the largest tidal range on December 8? In other words, what was the change in sea level between the second high tide and the second low tide?

8. What was the smallest tidal range on December 8? In other words, what was change in sea level between the first high tide and the first low tide? T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 4

Observations of the Tides

The tide charts below show the tides over a few days. Some interesting features stand out. Typically there are 2 high tides each day, but there are some days with only 1 high tide or 1 low tide (like April 14th in the chart below). (In some places, 1 high tide per day and 1 low tide per day is the normal pattern!) Along our coast, the time between high and low tide can vary from about 4 hours to nearly 8 hours.

On average, it takes about 6 hours and 10 minutes to go from high tide to low tide (and vice versa), so on most days it takes a little over 24 hours to complete the cycle. This means that tides typically get a little later each day, as can be seen in the charts below. Some days a tide may only be 20 minutes later than the previous day, and on other days a tide may be 2 hours later than the previous day. On average, tides are close to an hour later each day (about 50 minutes, to be a bit more precise).

T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 5

9. How many high tides are there each day (at most places in the world)?

10. How many low tides are there each day (at most places in the world)?

11. About how long does it take for the local ocean to change from high tide to low tide?

12. About how long does it take for the local ocean to change from low tide to high tide?

13. About how much time passes between high tides? In other words, about how long does it take for the local ocean to change from high tide to low tide and back to high tide again?

14. Do tides get earlier or later each day? About how much earlier or later? T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 6

Tides and the Moon: Spring Tides and Neap Tides

The moon changes appearance over the course of a month. Unlike the Sun, the Moon does not emit its own light. The Moon shines because it reflects light from the Sun. Just like the Earth has a daytime side facing the Sun and a night time side facing away from the Sun, there is a light-lit side of the Moon facing the Sun and dark side of the Moon facing away from the Sun. As the Moon orbits around the Earth each month, sometimes we see more of one side than the other side. We say that the Moon goes through different phases during a month. A full moon is when we see the side of the moon facing the Sun. A half moon is when we see part of the light side of the Moon and part of the dark side of the Moon. A new moon is when we see the dark side of the Moon.

Notice the link between tides and the phase of the Moon in the tide chart below showing an entire month of tides. High tides are highest and low tides are lowest during the full moon and the new moon, whereas high tides are not very high and low tides are not very low during a half moon. We call the large tidal range (the large difference between high and low tides) during full moon and new moon spring tides, and the small tidal range (the small difference between high and low tides) during half moons neap tides.

It takes about a week for the phase of the Moon to change, so tides switch from spring tides to neap tides in about a week. Notice that after the full moon there is another half moon, represented by the half moon off to the side of the chart. So, during a month spring tide conditions occur twice (once during the full moon and once during new moon) and neap tide conditions occur twice (during the two half moons).

T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 7

15. How are high tides different during spring tide conditions, very high or not very high?

16. How are low tides different during spring tide conditions, very low or not very low?

17. During which phase(s) of the Moon do spring tide conditions happen?

18. How are high tides different during neap tide conditions, very high or not very high?

19. How are low tides different during neap tide conditions, very low or not very low?

20. During which phase(s) of the Moon do neap tide conditions happen?

21. (a) How often do spring tide conditions occur each month?

(b) How often do neap tide conditions occur each month?

22. About how long does it take for the local ocean to change from spring tide conditions to neap tide conditions?

T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 8

The Phases of the Moon and the Motions of the Earth, Moon, and Sun

The Moon orbits the Earth once per month. In other words, it takes a month for the Moon to travel all the way around the Earth. (Just look at our word “month.” It includes the word Moon! Perhaps we should call it a moonth instead…just remember, you heard it here first.) The Moon shines at night because the Moon reflects the light of the Sun: The light bounces off of the Moon towards the Earth. Of course, only the part of the Moon facing the Sun receives light. The part of the Moon facing away from the Sun is dark. As the Moon orbits the Earth, sometimes we see more of the bright side, the part facing the Sun, and sometimes we see more of the dark side, the side facing away from the Sun. Full moon is when we only see the bright side of the Moon, and new moon is when we only see the dark side of the Moon. Half Moon is when we see equal amounts of the bright side and the dark side.

The diagram below shows the orbit of the Moon around the Earth during 1 month and the phase of the Moon at each position.

As the diagrams below show, full moon (picture #1) is when the Moon and Sun are on opposite sides of the Earth, so we see the bright side of the Moon. Notice the little stick-figure person in the picture. The person is on the side of the Earth facing away from the Sun, so it is night time for them (midnight, in fact). If the person looks up in the sky, they will see only the side of the Moon facing the Sun, a full moon.

The Moon reaches the half moon position (picture #2) about a week later as the Moon travels around the Earth. (At this point, it is ¼ of the way around. A quarter of a month is approximately a week.) Now the stick figure will see part of the light side of the Moon and part of the dark side of the Moon, a half-moon. T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 9

In about another week, the Moon and Sun are on the same side of the Earth (picture #3). At this point in time, the stick figure can only see the dark side of the Moon, a new moon. And about a week later and another ¼ orbit of the Moon later (picture #4), the stick figure sees some of the light side of the Moon and some of the dark side of the Moon, another half moon.

After about one more week and another ¼ orbit of the Moon, the Moon is back where it started. One complete cycle of the Moon through its phases takes place over a month (about 4 weeks).

The meaning of a day and a year

By the way, a day is the length of time it takes the Earth to spin all the way around one time. Daytime for us is when our part of the Earth is facing the Sun, and nighttime is when our part of the Earth is facing away from the Sun. A year is how long it takes the Earth to go all the way around the Sun one time.

23. How long does it take the Moon to orbit the Earth one time?

24. Why does the Moon shine at night?

25. During which phase of the Moon is the Moon brightest and the nighttime least dark?

26. During which phase of the Moon is the Moon and nighttime darkest? T. James Noyes, El Camino College Tides Unit I: Observations (Topic 7A-1) – page 10

27. Why does the Moon change phases, from new moon to full moon and back again?

28. How long does it take the Earth to rotate one time on its axis? (In other words, how long does it take the Earth to spin all the way around one time?)

29. How long does it take the Earth to orbit the Sun one time?

Why the Earth does not get in the way

If you look at picture #1, it seems like the Earth should be blocking light from the Sun, preventing a full moon from occurring. Similarly, if you look at picture #3, it seems like the person on the Earth should not be able to see the new moon, because the Earth is in the way, blocking their view.

This does not happen all the time because (1) the Moon is very far from the Earth, and (2) the Moon does not orbit the Earth over the Earth’s Equator, but instead at a steep angle to the Equator. As you can see in the diagram below, the person on the nighttime side of the Earth can see the Moon both when it is a full moon and a new moon.

Sometimes the Moon happens to be near the Equator when it is time for a full moon or new moon. When this happens, a full or partial eclipse will occur. During a full moon, the Earth blocks light from the Sun casting its shadow over the Moon. During a new moon, the Moon gets between the Earth and the Sun and Earth is in the Moon’s shadow.