Tides Unit I: Observations (Topic 7A-1) – page 1
Name:
Tides Unit I: Observations
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.
High Tide
Low Tide
Tidal Range
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?
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 (peaks, crests) in the curve indicate high tides, and low points (valleys, 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
tides 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.
feet
8
The difference in height between
6
successive high and low tides is
called the tidal range. In the
4
chart shown on the right,
High Tide
the largest tidal range in 6 feet,
2
the difference in height between
the 1st low tide (1 foot) and
Midnight 4
the 2nd high tide (7 feet).
am
(Tidal range = 7 feet – 1 foot = 6 feet.)
High Tide
Low Tide
Low Tide
8
am
Noon
4
pm
8 Midnight
pm
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
Time
Height
7. What was the largest tidal range
on this day? 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 this day? In other words,
what was change in sea level
between the first high tide
and the first low tide?
1st Low Tide
2nd High Tide
2nd Low Tide
Tides Unit I: Observations (Topic 7A-1) – page 3
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 and/or 1
low tide (e.g., April 14th on the next page). (In some places, this is normal!) 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 (and the chart shows) that
tides typically get a little later each day. Some days, a tide may only be 20 minutes later than the
previous day, 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).
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?
Tides Unit I: Observations (Topic 7A-1) – page 4
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?
Tides and 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 (difference between high and low tides) during full moon and
new moon “spring tides,” and the small tidal range (very little 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.
New
Moon
Half
Moon
Full
Moon
15. How are high tides different during spring tide conditions?
16. How are low tides different during spring tide conditions?
17. During which phase(s) of the Moon do spring tide conditions happen?
Tides Unit I: Observations (Topic 7A-1) – page 5
18. How are high tides different during neap tide conditions?
19. How are low tides different during neap tide conditions?
20. During which phase(s) of the Moon do neap tide conditions happen?
21. How often do spring tide conditions occur each month?
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?
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
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 with 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. As the picture below shows, full moon (#1) is when the Moon and
Sun are on opposite sides of the Earth, so we see the bright side of the Moon. The Moon reaches
the half moon position (#2) in about a week later as it travels around the Earth. (At this point, it
is ¼ of the way around. A quarter of a month is approximately a week.) In about another week,
the Moon and Sun are on the same side of the Earth at new moon (#3): we can only see the dark
side of the Moon.
Tides Unit I: Observations (Topic 7A-1) – page 6
E
New Moon
Spring Tides
Full Moon
Spring Tides
E
S
#1
S
#3
Half Moon
Neap Tides
E
Half Moon
Neap
Tides
E
S
#2
S
#4
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?
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?