When a star reaches 'opposition' with the Sun....

.....what is the technical term we use for this?

In other words, the point in the earth's passage along the ecliptic at
which the star in question is closest to the earth? Or the equivalent
to a planetary 'opposition'.

I understand the idea of culmination or transit of a meridian
underneath the star, but this can happen on more than one night.

Thanks for any help- I'm sure I'm being a bit dense.

In article
,
wrote:

....what is the technical term we use for this?

In other words, the point in the earth's passage along the ecliptic at
which the star in question is closest to the earth? Or the equivalent
to a planetary 'opposition'.

I understand the idea of culmination or transit of a meridian
underneath the star, but this can happen on more than one night.

As the Earth orbits about the sun, its distance to a star increases or
decreases ... depending, of course, on how close to the ecliptic the
star is. Polaris, for instance, would see even less change in the
distance than any other star. (And recall that that change in distance
is negligible to begin with.)

For the star top be closest to the Earth, it has to be directly opposite
the sun ... and that would happen at midnight. For any given star,
there's only one day (night) in the year on which this occurs. Three
months earlier or later, the same star's transit would happen six hours
later or earlier, and the sun-earth-star line would be two line segments
forming a right angle.

It helps to draw this stuff on a sheet of paper and do some basic
geometry. :-)

--
Timberwoof me at timberwoof dot com http://www.timberwoof.com
People who can't spell get kicked out of Hogwarts.

"Timberwoof" wrote...
in message
...
In article
,
wrote:

....what is the technical term we use for this?

In other words, the point in the earth's passage along the ecliptic at
which the star in question is closest to the earth? Or the equivalent
to a planetary 'opposition'.

I understand the idea of culmination or transit of a meridian
underneath the star, but this can happen on more than one night.

As the Earth orbits about the sun, its distance to a star increases or
decreases ... depending, of course, on how close to the ecliptic the
star is. Polaris, for instance, would see even less change in the
distance than any other star. (And recall that that change in distance
is negligible to begin with.)

For the star top be closest to the Earth, it has to be directly opposite
the sun ... and that would happen at midnight. For any given star,
there's only one day (night) in the year on which this occurs. Three
months earlier or later, the same star's transit would happen six hours
later or earlier, and the sun-earth-star line would be two line segments
forming a right angle.

It helps to draw this stuff on a sheet of paper and do some basic
geometry. :-)

--
Timberwoof me at timberwoof dot com http://www.timberwoof.com
People who can't spell get kicked out of Hogwarts.

That's all well and good, TW, but it doesn't answer
ultralazarus2's question. And i cannot do any better,
because i, too, don't know what the technical term is
for the astral opposition described.

It does raise an interesting question about using the
parallax method of measuring the distance to a star,
and i hope you're reading, Odysseus, because you
might know the answer...

When taking the parallax measurement, can you just
do it and then wait six months to take the second
measurement? OR...

Do you first have to determine the astral opposition
point of the star with the Sun, then wait nine months
to take the first parallax measurement, then wait six
months to take the second measurement?

IOW, how important is it to have the star at astral
opposition to the Sun as the CENTER of the parallax
measurements?

I hope i wrote that clearly enough. All the sites i can
find that describe the parallax method illustrate it by
showing the target star directly above the Earth and
the angles made when the Earth is three months
before the centerline on the right side of the Sun, and
three months after the centerline on the left side of
the Sun. So these illustrations depict the centerline
as going through the center of the Sun, the Earth and
the target star.

Unfortunately, none of the sites i've visited so far say
what the technical term is for that point of opposition
of the target star to the Sun.

happy days and...
starry starry nights!

--
Indelibly yours,
Paine Ellsworth

P.S.: "Hide not your talents, they for use were made.
What's a sun-dial in the shade?"
Benjamin Franklin

P.P.S.: http://yummycake.secretsgolden.com
http://garden-of-ebooks.blogspot.com
http://painellsworth.net

In article
,
"Painius" wrote:

"Timberwoof" wrote...
in message
...
In article
,
wrote:

....what is the technical term we use for this?

In other words, the point in the earth's passage along the ecliptic at
which the star in question is closest to the earth? Or the equivalent
to a planetary 'opposition'.

I understand the idea of culmination or transit of a meridian
underneath the star, but this can happen on more than one night.

As the Earth orbits about the sun, its distance to a star increases or
decreases ... depending, of course, on how close to the ecliptic the
star is. Polaris, for instance, would see even less change in the
distance than any other star. (And recall that that change in distance
is negligible to begin with.)

For the star top be closest to the Earth, it has to be directly opposite
the sun ... and that would happen at midnight. For any given star,
there's only one day (night) in the year on which this occurs. Three
months earlier or later, the same star's transit would happen six hours
later or earlier, and the sun-earth-star line would be two line segments
forming a right angle.

It helps to draw this stuff on a sheet of paper and do some basic
geometry. :-)

--
Timberwoof me at timberwoof dot com http://www.timberwoof.com
People who can't spell get kicked out of Hogwarts.

That's all well and good, TW, but it doesn't answer
ultralazarus2's question. And i cannot do any better,
because i, too, don't know what the technical term is
for the astral opposition described.

It does raise an interesting question about using the
parallax method of measuring the distance to a star,
and i hope you're reading, Odysseus, because you
might know the answer...

When taking the parallax measurement, can you just
do it and then wait six months to take the second
measurement? OR...

Do you first have to determine the astral opposition
point of the star with the Sun, then wait nine months
to take the first parallax measurement, then wait six
months to take the second measurement?

The second option would be the most wrong. As I explained, the
opposition we're talking about happens at midnight. So there are two
problems with taking the other reading six months later. First, the
measured parallax will be near zero ... if you can see the star, which
because of the second problem will not be visible.

The best time to take such a measurement for a star close to the
ecliptic is when it's an hour or so before sunrise on one side and an
hour or so after sunset on the other.

IOW, how important is it to have the star at astral
opposition to the Sun as the CENTER of the parallax
measurements?

That would maximize the parallax measurement. Optimum, if you can get
enough darkness at either end, is to have two right triangles.

I hope i wrote that clearly enough. All the sites i can
find that describe the parallax method illustrate it by
showing the target star directly above the Earth and
the angles made when the Earth is three months
before the centerline on the right side of the Sun, and
three months after the centerline on the left side of
the Sun. So these illustrations depict the centerline
as going through the center of the Sun, the Earth and
the target star.

Unfortunately, none of the sites i've visited so far say
what the technical term is for that point of opposition
of the target star to the Sun.

Opposition.

--
Timberwoof me at timberwoof dot com http://www.timberwoof.com
People who can't spell get kicked out of Hogwarts.

Even the diameter of the Earth's orbit doesn't really change the
distance to any star (well, except the Sun a tiny bit).

Saul Levy


On Sat, 15 Nov 2008 16:12:00 -0800 (PST),
wrote:

....what is the technical term we use for this?

In other words, the point in the earth's passage along the ecliptic at
which the star in question is closest to the earth? Or the equivalent
to a planetary 'opposition'.

I understand the idea of culmination or transit of a meridian
underneath the star, but this can happen on more than one night.

Thanks for any help- I'm sure I'm being a bit dense.

In article
,
"Painius" wrote:

"Timberwoof" wrote...
in message
...
In article
,
wrote:

....what is the technical term we use for this?

In other words, the point in the earth's passage along the ecliptic at
which the star in question is closest to the earth? Or the equivalent
to a planetary 'opposition'.

snip

For the star top be closest to the Earth, it has to be directly opposite
the sun ... and that would happen at midnight. For any given star,
there's only one day (night) in the year on which this occurs. Three
months earlier or later, the same star's transit would happen six hours
later or earlier, and the sun-earth-star line would be two line segments
forming a right angle.

snip

That's all well and good, TW, but it doesn't answer
ultralazarus2's question. And i cannot do any better,
because i, too, don't know what the technical term is
for the astral opposition described.

I don't know a term other than "solar opposition" for the position in
the sky, but the phenomenon itself is called "midnight culmination", and
the approximate date on which it recurs every year is often mentioned in
manuals &c.

It does raise an interesting question about using the
parallax method of measuring the distance to a star,
and i hope you're reading, Odysseus, because you
might know the answer...

When taking the parallax measurement, can you just
do it and then wait six months to take the second
measurement? OR...

Do you first have to determine the astral opposition
point of the star with the Sun, then wait nine months
to take the first parallax measurement, then wait six
months to take the second measurement?

I don't think so: if you can calculate the amount of the Earth's
displacement in its orbit, between any two reasonably widely separated
observations, and project that distance onto a line perpendicular to the
star's direction, you'll have your baseline for the triangulation. It
doesn't much matter when the observations are made: any time the star is
high enough that the effect of atmospheric refraction is small will do
fine. I suppose the 'ideal' pair of observations for simplicity and ease
of calculation would be one month before and one month after the
midnight-culmination date, because that way the baseline would already
be square and 1 AU long, but it's not at all difficult to deal with an
oblique baseline of some other length (as long as it's not too short).
Considering how finicky the measurements are, I imagine the best
strategy would be to collect as many observations as possible and
average the results, rather than going for a single 'ideal' pair. Unless
you already know the star's proper motion, you'll need observations
collected over a fairly long period -- several years at least -- to
distinguish the annual wobbles from secular travel.

--
Odysseus

"Odysseus" wrote in message...
news
In article
,
"Painius" wrote:
"Timberwoof" wrote...
in message
...
In article
,
wrote:

....what is the technical term we use for this?

In other words, the point in the earth's passage along the ecliptic at
which the star in question is closest to the earth? Or the equivalent
to a planetary 'opposition'.

snip

For the star top be closest to the Earth, it has to be directly
opposite
the sun ... and that would happen at midnight. For any given star,
there's only one day (night) in the year on which this occurs. Three
months earlier or later, the same star's transit would happen six hours
later or earlier, and the sun-earth-star line would be two line
segments
forming a right angle.

snip

That's all well and good, TW, but it doesn't answer
ultralazarus2's question. And i cannot do any better,
because i, too, don't know what the technical term is
for the astral opposition described.

I don't know a term other than "solar opposition" for the position in
the sky, but the phenomenon itself is called "midnight culmination", and
the approximate date on which it recurs every year is often mentioned in
manuals &c.

It does raise an interesting question about using the
parallax method of measuring the distance to a star,
and i hope you're reading, Odysseus, because you
might know the answer...

When taking the parallax measurement, can you just
do it and then wait six months to take the second
measurement? OR...

Do you first have to determine the astral opposition
point of the star with the Sun, then wait nine months
to take the first parallax measurement, then wait six
months to take the second measurement?

I don't think so: if you can calculate the amount of the Earth's
displacement in its orbit, between any two reasonably widely separated
observations, and project that distance onto a line perpendicular to the
star's direction, you'll have your baseline for the triangulation. It
doesn't much matter when the observations are made: any time the star is
high enough that the effect of atmospheric refraction is small will do
fine. I suppose the 'ideal' pair of observations for simplicity and ease
of calculation would be one month before and one month after the
midnight-culmination date, because that way the baseline would already
be square and 1 AU long, but it's not at all difficult to deal with an
oblique baseline of some other length (as long as it's not too short).
Considering how finicky the measurements are, I imagine the best
strategy would be to collect as many observations as possible and
average the results, rather than going for a single 'ideal' pair. Unless
you already know the star's proper motion, you'll need observations
collected over a fairly long period -- several years at least -- to
distinguish the annual wobbles from secular travel.

--
Odysseus

Thank you, Odysseus! That's fascinating!

happy days and...
starry starry nights!

--
Indelibly yours,
Paine Ellsworth

P.S.: "All faults may be forgiven of him who has
perfect candor." Walt Whitman

P.P.S.: http://yummycake.secretsgolden.com
http://garden-of-ebooks.blogspot.com
http://painellsworth.net