A friend asked me why his house, located in Gurgaon, India, received more morning sunlight in winter, and more evening sunlight in summer. He looked up the angles of sunrise & sunset in summer & winter.
Here's the sunrise and sunset angles for Gurgaon that was posted.
Sunrise : 63* NE
Sunset : 297* NW
Winters :
Sunrise : 116* SE
Sunset : 244* SW
I will deal with the direction, then the angles. Notice that the sun is depicted as rising and setting in the North East and North West respectively in summer and in the South East and South West in Winter. It doesn't change direction. It is simply that in summer, the sun rides much higher in the sky than in winter. As India is in the Northern hemisphere, the position of the sun is higher, i.e. more Northerly, than in winter, when it sits more to the South. That is to say, it follows a much bigger arc, which intuitively makes sense, as the sunrise to sunset time, i.e. the total hours of sunlight, are much longer in summer. That explains the NE, NW in summer and SE, SW in winter.
Now the degrees. This confirms what I said earlier- the arc traversed in summer is much bigger- 297-63=234 degrees than in winter- 244-116= 128 degrees. The sun sits lower, i.e. more to the South in the Northern hemisphere, hence the arc is flatter and lower.
While this explains why the sun should hang around for longer in summer evenings, it certainly doesn't explain why you enjoy sunnier mornings in winter. I looked into this as well. The workings are fascinating and made lovely reading. I would be happy to share if there is any interest at all.
It is commonly believed that the winter solstice in the Northern hemisphere is the day with the most delayed sunrise and the earliest sunset. Indeed, the play of seasons and longer-shorter day length in summer-winter is a function of the earth's inclination relative to it's geometric axis. This angle of inclination ( or "declination" as it's called) is about 23.4 degrees, so that the North pole always faces the sun in summer, while the South pole faces the sun in winter. The inclined axis doesn't change direction. It always faces the Polaris, which is why it's as "Constant as the Northern Star".
However, the earth also revolves around the sun. This orbit is slightly elliptical, so that in December, the earth is closest to the Sun, called Perihelion, while in June, it is at it's furthest, called aphelion. Due to a law of planetary motion, called Kepler's second law, the earth moves faster in its orbit at perihelion, i.e. in December, than at aphelion, i.e. in June. From the vantage point of somebody sitting in the Sun, the earth will appear to be moving almost 7% faster in December therefore*. From the earth however, the Sun's movement through the sky will appear delayed. Thus, towards the end of the year, the Sun is "late". Both sunrise and sunset are later than would be predicted by the earth's declination alone. The reverse occurs in June, albeit less accentuated. That is to say, the sun is "early". Both sunrise and sunset occur earlier than would be expected from the earth's declination alone.
Thus, there are two solar phenomena determining the sunrise-sunset cycle. The first is due to the earth's declination, and can be called the "geometric effect". The second is due to the effect of the elliptical axis, and is called the "clock effect".
In practical terms, what this means is that in December, the delayed sunrise from the geometric effect is augmented by the clock effect, while the earlier sunset from the geometric effect is countered by the clock effect. Thus, say at 40 degrees North, sunset starts to occur later in the day as early as Dec 8, rather than Dec 21, while sunrise starts to occur earlier much later- around Jan 6. The effect is around 16 minutes each way.
The reverse occurs in June. As the sun is "early", sunrise occurs earlier than expected even before June 21, while the peak in late sunset is delayed until July.
It's worth saying here that the Clock effect is invariant, regardless of latitude, while the Geometric effect is more pronounced the further away from the equator you go. At the equator, because the Clock effect predominates, sunset starts occurring later and later as early as November, while it's February before sunrise starts occurring earlier. Timewise, the Clock effect is least pronounced around the equinoxes, and most pronounced around the solstices.
Practically, this means that closer to the equator (Gurgaon 28 degrees North), the Geometric effect is minimal, and the Clock effect occurs for most of the variation in sunrise and sunset. At the equator, this amounts to only around 30 minutes between summer and winter.
In more northerly latitudes ( say Nottingham 53 degrees North), the Geometric effect becomes more pronounced, accounting for the very short days in winter.
Now back to my friend's query. Why are the mornings sunnier in winter? The clock effect should not contribute more than 16 minutes to a change in the time of sunrise, but the problem with invoking it is that sunrise occurs later due to Clock effect in winter, chiming in with the Geometric effect. Hence that doesn't explain the winter morning Sun. The most likely explanation is therefore either less cloud cover in Winter or more likely a South East facing window, which captures most of the sunshine that's available. This latter explains why South facing gardens are prized in more northerly countries such as the UK.
* Kepler's second law - sometimes referred to as the law of equal areas - describes the speed at which any given planet will move while orbiting the sun. The speed at which any planet moves through space is constantly changing. A planet moves fastest when it is closest to the sun and slowest when it is furthest from the sun. Yet, if an imaginary line were drawn from the center of the planet to the center of the sun, that line would sweep out the same area in equal periods of time. For instance, if an imaginary line were drawn from the earth to the sun, then the area swept out by the line in every 31-day month would be the same. (http://www.physicsclassroom.com/class/circles/Lesson-4/Kepler-s-Three-Laws)
Here's the sunrise and sunset angles for Gurgaon that was posted.
Sunrise : 63* NE
Sunset : 297* NW
Winters :
Sunrise : 116* SE
Sunset : 244* SW
I will deal with the direction, then the angles. Notice that the sun is depicted as rising and setting in the North East and North West respectively in summer and in the South East and South West in Winter. It doesn't change direction. It is simply that in summer, the sun rides much higher in the sky than in winter. As India is in the Northern hemisphere, the position of the sun is higher, i.e. more Northerly, than in winter, when it sits more to the South. That is to say, it follows a much bigger arc, which intuitively makes sense, as the sunrise to sunset time, i.e. the total hours of sunlight, are much longer in summer. That explains the NE, NW in summer and SE, SW in winter.
Now the degrees. This confirms what I said earlier- the arc traversed in summer is much bigger- 297-63=234 degrees than in winter- 244-116= 128 degrees. The sun sits lower, i.e. more to the South in the Northern hemisphere, hence the arc is flatter and lower.
While this explains why the sun should hang around for longer in summer evenings, it certainly doesn't explain why you enjoy sunnier mornings in winter. I looked into this as well. The workings are fascinating and made lovely reading. I would be happy to share if there is any interest at all.
It is commonly believed that the winter solstice in the Northern hemisphere is the day with the most delayed sunrise and the earliest sunset. Indeed, the play of seasons and longer-shorter day length in summer-winter is a function of the earth's inclination relative to it's geometric axis. This angle of inclination ( or "declination" as it's called) is about 23.4 degrees, so that the North pole always faces the sun in summer, while the South pole faces the sun in winter. The inclined axis doesn't change direction. It always faces the Polaris, which is why it's as "Constant as the Northern Star".
However, the earth also revolves around the sun. This orbit is slightly elliptical, so that in December, the earth is closest to the Sun, called Perihelion, while in June, it is at it's furthest, called aphelion. Due to a law of planetary motion, called Kepler's second law, the earth moves faster in its orbit at perihelion, i.e. in December, than at aphelion, i.e. in June. From the vantage point of somebody sitting in the Sun, the earth will appear to be moving almost 7% faster in December therefore*. From the earth however, the Sun's movement through the sky will appear delayed. Thus, towards the end of the year, the Sun is "late". Both sunrise and sunset are later than would be predicted by the earth's declination alone. The reverse occurs in June, albeit less accentuated. That is to say, the sun is "early". Both sunrise and sunset occur earlier than would be expected from the earth's declination alone.
Thus, there are two solar phenomena determining the sunrise-sunset cycle. The first is due to the earth's declination, and can be called the "geometric effect". The second is due to the effect of the elliptical axis, and is called the "clock effect".
In practical terms, what this means is that in December, the delayed sunrise from the geometric effect is augmented by the clock effect, while the earlier sunset from the geometric effect is countered by the clock effect. Thus, say at 40 degrees North, sunset starts to occur later in the day as early as Dec 8, rather than Dec 21, while sunrise starts to occur earlier much later- around Jan 6. The effect is around 16 minutes each way.
The reverse occurs in June. As the sun is "early", sunrise occurs earlier than expected even before June 21, while the peak in late sunset is delayed until July.
It's worth saying here that the Clock effect is invariant, regardless of latitude, while the Geometric effect is more pronounced the further away from the equator you go. At the equator, because the Clock effect predominates, sunset starts occurring later and later as early as November, while it's February before sunrise starts occurring earlier. Timewise, the Clock effect is least pronounced around the equinoxes, and most pronounced around the solstices.
Practically, this means that closer to the equator (Gurgaon 28 degrees North), the Geometric effect is minimal, and the Clock effect occurs for most of the variation in sunrise and sunset. At the equator, this amounts to only around 30 minutes between summer and winter.
In more northerly latitudes ( say Nottingham 53 degrees North), the Geometric effect becomes more pronounced, accounting for the very short days in winter.
Now back to my friend's query. Why are the mornings sunnier in winter? The clock effect should not contribute more than 16 minutes to a change in the time of sunrise, but the problem with invoking it is that sunrise occurs later due to Clock effect in winter, chiming in with the Geometric effect. Hence that doesn't explain the winter morning Sun. The most likely explanation is therefore either less cloud cover in Winter or more likely a South East facing window, which captures most of the sunshine that's available. This latter explains why South facing gardens are prized in more northerly countries such as the UK.
* Kepler's second law - sometimes referred to as the law of equal areas - describes the speed at which any given planet will move while orbiting the sun. The speed at which any planet moves through space is constantly changing. A planet moves fastest when it is closest to the sun and slowest when it is furthest from the sun. Yet, if an imaginary line were drawn from the center of the planet to the center of the sun, that line would sweep out the same area in equal periods of time. For instance, if an imaginary line were drawn from the earth to the sun, then the area swept out by the line in every 31-day month would be the same. (http://www.physicsclassroom.com/class/circles/Lesson-4/Kepler-s-Three-Laws)
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