This page consists of questions and exercises about the subjects of The Natural Year Circular Almanac, along with some links to other good sites, including the sources of most of the data. The questions require a little bit of thought and understanding, while the exercises are designed to test the ability to read graphical information. It may be helpful to view a high-quality pdf image of The Natural Year in an Adobe Acrobat reader while you read these questions.
The diagram (not to scale) shows the orbits of Venus, the Earth, and Mars around the sun, as well as some positions along the orbit that Venus and Mars have at some time.
The orbit of Venus is closer to the sun than that of the Earth. Because of this, Venus always appears to us to be close to the sun in the sky, appearing in the east before sunrise, or in the west after sunset. For Venus to be in eastern sky at sunset, it would have to be farther from the sun than the earth is.
Mars is farther from the sun than the Earth is, and so sometimes it is in the east when the sun is in the west, and vice versa. Sometimes, when Mars is on the far side of the sun from us, it appears close to the sun in the sky.
The Annual Editions of The Natural Year show the positions of Mars and Venus in the sky throughout the year.
Retrograde motion is the apparent change in the motion of the planets as we see them from the Earth, usually, the planets move from west to east across the sky as the year passes, but sometimes they seem to move from east to west. The diagram at right shows the retrograde motion of Mars during June through August (J-F-M-A-M-J-J-A) of 1999 as it appeared in The Natural Year Circular Almanac. The diagram below shows why this happens.
The diagram shows the sun, Earth, Mars and a star. It is fairly generic, since the distance and time scales are somewhat off, and it does not represent any particular star. However, it works to show the principles involved. Here we are looking 'up' at the solar system, since this orientation matches that used in the star chart of The Natural Year. In this view the planets rotate clockwise around the sun.
The line drawn from January on Earth, through January on Mars, out towards the star show the apparent position of Mars from the Earth in January. Mars appears to be 'above' the star, this shows up as farther west in the sky. In April, both the Earth and Mars have moved along their orbits, and the apparent position of Mars is now 'below' (east of) the star. This is the general trend: the apparent position of Mars moves east across the sky. However, by July things have changed. The apparent position of Mars has moved back 'above' (west of) the star. This is retrograde motion. What has happened is that, in this diagram, the Earth is 'passing' Mars between April and July, and the Mars seems to go backwards.
The Annual Editions of The Natural Year show the positions of Mars and Venus in the sky throughout the year.
Venus is always relatively close to the sun because its orbit is closer to the sun than our own. Venus is the Evening Star when it is east of the sun. Then Venus sets after the sun, and we see it in the evening sky, while it rises in the morning after the sun and we can't see it in the daytime sky. It is the Morning Star when it is to the west of the sun. Then it rises before the sun and we see it in the morning, while it also sets before the sun and we do not see it in the evening. Sometimes Venus is behind the sun, or directly in front of it, or too close to it to be seen.
Using Annual Editions of The Natural Year, you can tell the relative position of Venus to the sun at any time of the year. Find the position of Venus for the month in question by following the path of Venus (the yellow line in the star chart) until you hit the letter for that month. Compare the clockwise position of this point with clockwise position of the same month around the outer ring of The Natural Year. For example, at the beginning of March, Venus is at about 3:00 o'clock, while the label for March is at about 2:00 o'clock. Thus Venus is farther east than the sun at this time, and will be the Evening Star. When does this change?
Question and answer submitted by Karen E. Ryan, 7th Grade Science Teacher, Raymore-Peculiar Middle School
The earth rotates around its own axis, an imaginary line between the north and south poles. The axis currently points quite closely to the star Polaris, and so we (in the northern hemisphere) see Polaris as the North Star. However, the direction of the axis is not constant. The Earth has a slow 'wobble' so the axis does not always point to the same spot in the sky. Most of the time the axis does not point to a star, and there is no North Star. Sometimes the axis points to a different star. In about 13,000 years the star Vega in the constellation Lyra will be the North Star. It takes the Earth about 26,000 years to complete one full wobble.
In the start chart in The Natural Year, Polaris is in the center, showing its current status as the North Star.
As the days grow longer in spring, temperatures warm. Not only is the sun up longer, it is also higher in the sky, so the amount of heat we receive increases greatly, causing temperatures to rise. The rise in temperatures is somewhat delayed, however, and the highest average temperatures occur a month or so after the longest day.
A similar effect is seen during autumn. The days get shorter, the sun is lower in the sky, and temperatures cool. Again, there is a delayed effect, and the lowest temperatures occur after the shortest day.
The detail from The Natural Year at right includes the summer solstice, when the longest day of the year occurs. The yellow band represents daylight. Note that the average high and low temperatures (red and blue) are high but still rising.
The small globe diagram at the bottom of the image shows how the northern hemisphere is getting maximum sunlight at this time. See the legend or the pdf image for more details on the reading the information in The Natural Year.
Fog occurs when droplets of water form in the air, condensing around tiny particles of dust. The rate of condensation depends on the amount of water the air can hold, and the higher the temperature, the more water it can hold. Thus fog tends to form as the air cools in the evening, and dissipates as the air warms during the day.
The Natural Year shows morning and afternoon humidity, which is the relative amount of water that the air can carry. We can see that the relative humidity is usually higher in the morning when temperatures are lower.
The phase of the moon and the time of moonrise are completely determined by the relative positions, of the Earth, sun and moon. Remember that the moon is always half lit by the sun and half dark. Some times we see the lit part, sometimes the dark part, usually some of both.
| When the Earth is directly between the sun and the moon, the moon appears opposite the sun in the sky, thus moonrise is at sunset and moonset is at sunrise. At this time the half of the moon that is lit by the sun is completely visible to us, since when we are looking at the moon the sun is behind us. Thus the moon is full when it rises at sunset. |
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| Full Moon |
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When the moon is between the Earth and the sun, we see it rise and set at the same time as the sun. Also, we can't see the half of the moon that is lit because it is facing away from us towards the sun. This is the new moon. |
| New Moon |
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| A half moon occurs when the moon is at a right angle to the earth-sun line. |
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| Half Moon |
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What does this suggest about the phases of the moon during lunar and solar eclipses?
The phase of the moon for each day is illustrated in The Natural Year, as is the time of moonrise and moonset. See how the pattern of the moonrise/set bar as the lunar month passes is correlated to the image of the phase.
Submitted by Mark Ruede
Since a new moon occurs when the moon is between the Earth and the sun, it might seem that the moon should block out the sun when this happens, causing a solar eclipse. Likewise, during a full moon, when the Earth is between the moon and the sun, why doesn't the Earth's shadow fall on the moon, causing a lunar eclipse?
The reason this does not happen is that the moon's orbit around the Earth is tilted compared to the Earth's orbit around the sun. So when we say the the moon is 'between' the Earth and sun, or the Earth is 'between' the moon and sun, we are usually being approximate, and the three objects do not lie in a straight line. The moon's tilted orbit around the Earth does cross the plane of the Earth's orbit around the sun. If this happens during a new moon or full moon, there will be a solar or lunar eclipse.
The phase of the moon for each day is illustrated in The Natural Year, as is the time of moonrise and moonset. See how the pattern of the moonrise/set bar as the lunar month passes is correlated to the image of the phase.
Submitted by Mark Ruede
The Earth is much bigger than the sun and so it casts a bigger shadow. Thus it takes more precise conditions for the moon to cover the sun than for the earth.
The orbit of the moon around the Earth is tilted with respect to Earth's equator. This causes the moon to rise farther north at some times and farther south at others. In the northern hemisphere, we see takes a longer path, and is seen in the sky for a longer time, when it rises farther north.
It could just as well be said that the Earth is tilted with respect to the orbit of the moon, in the same way the Earth is tilted with respect to its orbit around the sun. Thus the changing length of the days over the year is similar to the changing duration of the moon in the sky over the month.
In The Natural Year, the changing length of the moon's stay in the sky is clearly apparent in the outer band showing sunrise/set and moonrise/set.
Submitted by Emily Casey
In a simplified view the answer would be no, since the original force driving the high tides is the gravitational pull of the moon, and this has two peaks a day, one when the moon is directly overhead and one when the moon is directly on the opposite side of the Earth. In fact, since the moon is in orbit around the Earth, it does not quite complete a full turn in one day, so the high tides are more than twelve hours apart, and some days only have one high tide.
The simple view is often not good enough, however. Tides are affected by local factors, especially the shape of the ocean bottom and the coastlines. This can cause complex interactions that are hard to predict, and in fact it may happen in some places that several ebb and flow cycles occur in a single day, causing more than two high tides.
In The Natural Year, the ebb and flow of the tides is shown in blue, just inward from the moon phase band.
Answer submitted by Arye Hassen, Hoffit, Israel.
The charted sea level or chart datum is usually the lowest level to which the tide falls. This is for the safety of ships.
In The Natural Year, the ebb and flow of the tides is shown in blue, just inward from the moon phase band. The amount of rise and fall is shown on the grid overlaid on the blue tide line.
The circle represents the orbit of the Earth around the sun. In 365 days, the Earth does not quite complete one full orbit - it is about 1/4 of a days travel short. This is shown by the gap in the Perpetual Edition of the chart. It is why there is a leap year every 4 years. In 2000, a leap year, there are 366 days, and there is an overlap instead of a gap. The Natural Year handles this by squeezing the 366th day into the gap.
Question submitted by Lauren Cohen.
As the amount of daylight changes throughout the year, the rate of change varies also. Near the equinoxes the amount of change from day to day is large, more than 2 minutes per day in Kansas City. Near the solstices the amount of change from day to day is small, dropping to almost zero at the solstices. The overall pattern is an approximately sinusoidal curve, resulting from the approximately circular orbit of the Earth around the sun.
In The Natural Year, the rising and setting of the sun is shown in the in yellow in the 24 Hour Day band.
The Earth's north-south axis is not perpendicular to the plane of the Earth's orbit around the Sun. Because of this, sometimes during the year the northern hemisphere is pointing more towards the sun than the southern hemisphere, and at other times the situation is reversed. When the northern hemisphere is pointing more towards the sun, daylight hours are increased, and the Sun is higher in the sky. This is summer. Winter occurs when the northern hemisphere is pointing away from the Sun, and days are shorter and the Sun is lower in the sky. In the southern hemisphere the opposite seasons occur.
In The Natural Year, the orientation of the Earth with respect to the Sun is shown, along with the length of the day and the average temperatures.
This detail from The Natural Year shows the moon phases and tides over a 23 day period. (This is not a full lunar month, just a convenient size for the image.) Notice how the highs and lows of the tide are more pronounced when the moon is full or when the moon is new. The tides are least pronounced when the moon is half full.
This is because the tides are caused by the gravitational pull of the moon. When the moon is overhead, it pulls the water in the ocean towards it, causing high tide. When the moon is on the opposite side of the earth from us, it pulls the Earth away from the ocean, again causing a high tide.
When the moon, Earth, and sun are aligned, the sun's gravitational pull is added to that of the moon, accentuating the tidal flow. This happens when the moon is full (the Earth is directly between the sun and the moon), and when it is new (the moon is directly between the sun and the Earth).
Question and answer submitted by Billy DeLeo, oceanographer
First of all, different harbors around the globe have different types of tides, and the type of tide is not only dependent on the influence of the moon, but also on the geometry of the harbor. Long Island Sound (the source of the tidal data for the North East version of the almanac) has what is called a "semidiurnal" type tide, which means that there are two highs and two lows each day, just about. Another type is the "mixed" type, which is primarily semidiurnal, but with a strong diurnal inequality. A third is the diurnal type, which has one high and one low each day.
To answer the question, the strength of the inequality tends to increase with increasing declination of the moon. In other words, it tends to be a minimum when the moon passes over the equator.
In The Natural Year, the ebb and flow of the tides is shown in blue, just inward from the moon phase band.
Submitted by Madison Berg - 4 years old! (I've tried to phrase the answer in terms a 4 year old can understand.)
The sun rises and sets every day. In the morning, the sun rises, and we can see the sun all day long. This is daytime. In the evening, the sun sets, and we can't see it any more. This is night.
The moon rises and sets like the sun, so sometimes we can see it in the sky and sometimes we can't. However, the moon does not rise and set at the same time as the sun. So, sometimes the sun is up by itself, and we can see the sun but not the moon. Sometimes the moon is up by itself, and we see the moon but not the sun, at night. And sometimes both are up at the same time, and we can see the moon in the daytime.
In The Natural Year, the 24 Hour Day band shows the times the moon is in the sky with pink bars.
Question and answer submitted by Denise Berg - Middle school science teacher
January 1st is always the 1st day of the year, but December 31st is not always the 365th. During leap years, days following February 29th fall one day later in the year than they do in other years.
In The Natural Year, the day of the year is shown in the outer section of the 24 Hour day band.
Question and answer submitted by Kent Ellis, Physics Teacher, Flagstaff High School, Flagstaff, AZ.
Equinox is from the Latin for 'equal night', and indicates the time when daylight and darkness are equal. This occurs because the sun is at a right angle to Earth's axis, and the northern and southern hemispheres get the same exposure to the sun. This suggests that daytime and darkness are of equal duration at the equinoxes.
However, we get some bonus sunlight because the Earth's atmosphere refracts the sunlight, bending it a small amount around the nighttime side. The causes sunrise to be little earlier than it would be otherwise, and sunset to be a little later. So at the exact time of the equinoxes the days are longer than the nights for both hemispheres.
The days of 12-hours of daylight and 12-hours of darkness occur about five days after the autumnal equinox and five days before the vernal equinox.
In the diagram sunlight is coming from the left, lighting the left half of the Earth. The sunlight is refracted as it hits the Earth's atmosphere. The equator line points to the sun, because at the time of the equinoxes the sun is in the plane of the equator. The diagram exaggerates the depth of the atmosphere, the amount of refraction, and the size of the 'extra' daylight region.
Answer extended by Joe Portney of Navsense:
Another factor that increases the length of daylight is that the sun is a large object, and it takes time for it to rise and set completely. Daylight starts as soon as we see the top edge of the sun above the horizon, giving us a little extra compared to what we would see if the sun was a point source of light. A similar situation occurs at sunset. Altogether, we get extra daylight equal to the amount of time it takes the sun to rise or set completely.
In The Natural Year, the equinoxes and solstices are shown in diagrams at the appropriate locations around the year.
Question and answer submitted by Bryan Piersol, Cullowhee, North Carolina
During summer solstice, on June 21, the north polar end of the earth's axis is tilted at its maximum angle of 23.5 degrees toward the sun. The Tropic of Cancer, an imaginary line circling the globe at 23.5 degrees North, is the point on the earth where solar energy is most intense during summer solstice. Along this line the sun is directly overhead around noon at the time of the solstice. Farther north than this the sun is never directly overhead.
During winter solstice, the conditions are exactly reversed. On December 21, the south polar end of the earth's axis is tilted toward the sun at its maximum angle of 23.5 degrees. The Tropic of Capricorn is located at 23.5 degrees South, and is the point on the earth where solar energy is most intense during winter solstice.
In The Natural Year, the equinoxes and solstices are shown in diagrams at the appropriate locations around the year.
Question submitted by Diane Jensen, Spokane, Washington.
A Blue moon is the third full moon in a season with four full moons. A very common misconception is that a Blue Moon is the second full moon in a month. For a detailed discussion of this topic, see an article in Sky and Telescope magazine.
In The Natural Year, the phase of the moon is shown for every day of the year.
Question submitted by Jim Rosenberg, Wassenaar, The Netherlands.
© 1998-2006 Joseph B. Casey