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13 Solar Astronomy
Compare the images in hydrogen-alpha to those in white light and monitor how
the sunspots and other transient phenomena, such as prominences, change over the
course of the observations.
Most FITS manipulation software can draw a profile, for example, ATP. Use this
feature to determine the brightness gradient of the Sun across its surface.
As with previous practicals, you should write up your report in standard laboratory
style. You should discuss the different structures you see and compare how their
appearance differs between hydrogen-alpha and white light. You should determine
the rotation rate of the Sun and comment on how, if at all, it changes with latitude.
The brightness gradient over the surface of the Sun is known as limb darkening. You
should investigate what is causing this effect and whether it is time variable.
13.5 Eclipse Observing
It is a remarkable coincidence that the ratio between the diameters of the Moon
and the Sun and the ratio of the distances from Earth to the Sun and the Moon are
almost identical, meaning that both the Sun and the Moon appear to have an angular
size of half a degree. The orbit of the Moon is inclined by 5.145 deg to the ecliptic,
resulting in the apparent paths of the Moon and the Sun intersecting at two points,
known as nodes. When the Sun and Moon are both positioned at opposite nodes,
a lunar eclipse occurs, whereby the Earth’s shadow falls onto the Moon, leaving it
illuminated by only the light refracted through the Earth’s atmosphere. This gives
the Moon a red appearance, leading in recent years to the popular but misleading
term “blood moon.”
When the Sun and the Moon are at the same node, a solar eclipse occurs. Due to
the smaller size of the Moon’s shadow when compared to the Earth’s, solar eclipses
are less common at any one location, are briefer, and can be seen over a wider area
of the Earth than solar eclipses.
Solar eclipses come in four broad types: A total eclipse occurs when the shadow
of the Moon completely obscures the Sun, allowing the much fainter solar corona
to be visible. This is visible to all observers in the umbra section of the Moon’s
shadow. Outside of the umbra, within the penumbra, observers will see a partial
solar eclipse. This can usually be seen from a larger part of the Earth than the total
eclipse. However, some eclipses can be observed only as a partial eclipse, because the
umbra never intersects the Earth’s surface. Partial eclipses, although more common at
any one location, are considerably less impressive than total eclipses. As the Moon’s
orbit around the Earth is imperfectly circular, the apparent diameter of the moon
can change by about 6%. When the Moon is at its apparent smallest diameter, it
is not large enough to cover the Sun. This can result in an annular eclipse, which
occurs when the Sun and Moon occupy the same node, but the apparent size of the
Moon is smaller than that of the Sun. This results in the Sun appearing as a very
bright ring, or annulus, surrounding the eclipsing disk of the Moon. An observer
must be in the antumbar in order to see the annulus. Much rarer is the hybrid, or
13 Solar Astronomy
Compare the images in hydrogen-alpha to those in white light and monitor how
the sunspots and other transient phenomena, such as prominences, change over the
course of the observations.
Most FITS manipulation software can draw a profile, for example, ATP. Use this
feature to determine the brightness gradient of the Sun across its surface.
As with previous practicals, you should write up your report in standard laboratory
style. You should discuss the different structures you see and compare how their
appearance differs between hydrogen-alpha and white light. You should determine
the rotation rate of the Sun and comment on how, if at all, it changes with latitude.
The brightness gradient over the surface of the Sun is known as limb darkening. You
should investigate what is causing this effect and whether it is time variable.
13.5 Eclipse Observing
It is a remarkable coincidence that the ratio between the diameters of the Moon
and the Sun and the ratio of the distances from Earth to the Sun and the Moon are
almost identical, meaning that both the Sun and the Moon appear to have an angular
size of half a degree. The orbit of the Moon is inclined by 5.145 deg to the ecliptic,
resulting in the apparent paths of the Moon and the Sun intersecting at two points,
known as nodes. When the Sun and Moon are both positioned at opposite nodes,
a lunar eclipse occurs, whereby the Earth’s shadow falls onto the Moon, leaving it
illuminated by only the light refracted through the Earth’s atmosphere. This gives
the Moon a red appearance, leading in recent years to the popular but misleading
term “blood moon.”
When the Sun and the Moon are at the same node, a solar eclipse occurs. Due to
the smaller size of the Moon’s shadow when compared to the Earth’s, solar eclipses
are less common at any one location, are briefer, and can be seen over a wider area
of the Earth than solar eclipses.
Solar eclipses come in four broad types: A total eclipse occurs when the shadow
of the Moon completely obscures the Sun, allowing the much fainter solar corona
to be visible. This is visible to all observers in the umbra section of the Moon’s
shadow. Outside of the umbra, within the penumbra, observers will see a partial
solar eclipse. This can usually be seen from a larger part of the Earth than the total
eclipse. However, some eclipses can be observed only as a partial eclipse, because the
umbra never intersects the Earth’s surface. Partial eclipses, although more common at
any one location, are considerably less impressive than total eclipses. As the Moon’s
orbit around the Earth is imperfectly circular, the apparent diameter of the moon
can change by about 6%. When the Moon is at its apparent smallest diameter, it
is not large enough to cover the Sun. This can result in an annular eclipse, which
occurs when the Sun and Moon occupy the same node, but the apparent size of the
Moon is smaller than that of the Sun. This results in the Sun appearing as a very
bright ring, or annulus, surrounding the eclipsing disk of the Moon. An observer
must be in the antumbar in order to see the annulus. Much rarer is the hybrid, or