A solar chart is the standard workaround for a missing birth time, and it is also the most frequently misread one. The idea is straightforward: put the Sun on the first house cusp and rebuild the houses from there. It gives you twelve houses and a tidy wheel, which is exactly why people forget that the wheel is a construction, not a measurement.
Take the Sun's exact degree and sign at birth. Set that degree on the first house cusp, then count forward in whole signs: the sign after the Sun's goes on the second cusp, and so on around the wheel. Because each sign occupies exactly one house, this is a whole sign approach. No Placidus or Koch calculation is involved, and latitude drops out of the picture entirely.
The result is a chart with a determinate Ascendant, twelve cusps, and complete house placement for every planet. Every birthday produces exactly one solar chart, so two people born on the same date get the same house structure. That last point is the tell. House structure that does not vary between people is not describing anything about the individual.
It gives you a coherent framework when the alternative is nothing. Solar charts are used widely in solar return work, where the Sun's position genuinely is the reference point and the method is well developed. Transposed to natal work, they let you talk about which sign rules which arena of life in a consistent way.
It also handles the big picture of a chart's shape. If you have six planets in one sign, the solar chart shows you where the emphasis clusters in a way that a bare list of placements does not. And because it is whole sign, it sidesteps the quadrant house debates entirely, which some traditions prefer anyway.
It cannot tell you your rising sign. The first house of a solar chart is the Sun's sign, not the sign that was physically rising. For most people those are different, and for some people they are opposite. Reading the solar first house as the Ascendant is the most common error in this whole area.
It cannot tell you anything about angularity. A planet conjunct the real Ascendant behaves very differently from one sitting in a constructed first house, and the solar chart cannot distinguish them. It also loses the distinction between the Midheaven and the tenth house cusp, which in quadrant systems are usually not the same point.
A noon chart uses a real clock time and real astronomy. You set the time to 12:00, compute the actual Ascendant for that moment at the birth location, and get real cusps that happen to be wrong by up to twelve hours. A solar chart discards the clock entirely and uses the Sun as a stand-in anchor.
The noon chart has the advantage of being a real chart for a real moment, so its internal geometry is valid. Its disadvantage is that noon might be close to your birth time or as far from it as twelve hours, and you have no way to tell which. The solar chart has the advantage of being reproducible and unambiguous, and the disadvantage of never having been true of anyone.
Label it. Every time you write a solar chart interpretation, say that the houses are constructed from the Sun and are not your actual houses. That single sentence prevents a reader from walking away believing they have a Leo Ascendant when they have a solar first house in Leo.
Then ask what you actually want from the chart. If you want sign-level psychology, you do not need a solar chart at all. If you want a sense of which areas of life a planet's sign might express through, a clearly labeled solar chart gives you a usable scaffold while you look for the birth record or consider rectification.